Anatomy of a Data Science Software Toolkit That Uses Machine Learning to Aid ‘Bench-to-Bedside’ Medical Research—With Essential Concepts of Data Mining and Analysis Explained

Abstract

Data science and machine learning are buzzwords of the early 21st century. Now pervasive through human civilization, how do these concepts translate to use by researchers and clinicians in the life-science and medical field? Here, we describe a software toolkit, just large enough in scale, so that it can be maintained and extended by a small team, optimised for problems that arise in small/medium laboratories. In particular, this system may be managed from data ingestion statistics preparation predictions by a single person. At the system’s core is a graph type database, so that it is flexible in terms of irregular, constantly changing data types, as such data types are common during explorative research. At the system’s outermost shell, the concept of ’user stories’ is introduced to help the end-user researchers perform various tasks separated by their expertise: these range from simple data input, data curation, statistics, and finally to predictions via machine learning algorithms. We compiled a sizable list of already existing, modular Python platform libraries usable for data analysis that may be used as a reference in the field and may be incorporated into this software. We also provide an insight into basic concepts, such as labelled-unlabelled data, supervised vs. unsupervised learning, regression vs. classification, evaluation by different error metrics, and an advanced concept of cross-validation. Finally, we show some examples from our laboratory using our blood sample and blood clot data from thrombosis patients (sufferers from stroke, heart and peripheral thrombosis disease) and how such tools can help to set up realistic expectations and show caveats.

1. Introduction

Coronary artery disease (CAD), acute ischemic stroke (AIS), peripheral artery disease (PAD) are cardiovascular diseases and represent the leading morbidity and mortality causes globally [1]. The acute tissue damage is mostly due to thrombi occluding the supplying arteries [2]. The lysis susceptibility and stability of these thrombi ultimately determines the fate of the patient [3]. Can we tell more from their structure and from common blood test data of patients? Can we predict the diseases from this data? Even better, can we predict it before disease onset? These are questions that we address with the help of the data analysis approach described in this paper.

2. Data Science Project Organisation—Industry ’Best Practices’

As with any project, the question arises: where to begin? This is especially so with complex, data-driven projects, such as the ‘bench-to-bedside’ projects often seen in the medical and life science field. Fortunately, this problem has been seen before and some solutions, or rather, guidelines were devised. Industrial Micro Machines (IBM) researchers and automotive engineers (Daimler-Chrysler) did indeed face the same problem in the 1990s. The resultant guidelines were standardized in the CRISP-DM standard which stands for Cross Industry Standard Process for Data Mining [4]. The model, illustrated in Figure 1, consists of six steps [5].

• Business understanding. In the medical field, we would need to determine if the project is for research purposes only. Do we have obvious clinical implications (with rigorous safety, regulatory and legal requirements)? Do we eye business potential—which in turn may need strict documentation and reporting?
• Data understanding. In the medical and life science field, we often have to gather data via experiments ourselves, and the quality of this data will be crucial. (In the era of big data, many projects may rely on published open data sources.) What data can we gather and in what amount? Can we collect enough in a realistic timeframe? Will the data be sufficient for the questions asked? Do we have good recipes so that the data is reproducible over large timespans, with changing personnel?
• Data preparation. We must gather data from handwritten notes or other sources, collect them in machine-readable form, and normalize or otherwise standardize them. Watch out for reproducibility issues often encountered with difficult experiments. Can our data be quantified and compared across different investigators and with human subjects involved? Even standard laboratory tests may differ in testing methodology. With human subjects, legal issues also arise privacy rights need to be respected, and data anonymized.
• Modelling. We need to use our data—somehow. Do we have continuous variables or categorical ones? Do we have an initial hypothesis to test, or do we need methods that are suitable for ‘walking in the dark’?
• Evaluation. We need to use metrics by which we quantify ‘successes. Do we have concise research questions so that an answer to them can be made? Are we descriptive or do we have new hypotheses as a result? Without clear questions, no metrics will be satisfactory, as metrics are often meaningless themselves. Can we compare our work to that of others via the metrics?
• Deployment. Pure research may result in scientific outputs, such as presentations or scientific papers or openly published datasets. However, does our research yield tangible results such as new scientific hypotheses? Does our research have clinical implications?

This is the model we will follow and around which we build our homemade software, which will be described in more detail in the next sections.

2.1. Software Architecture—Incorporation of ‘Best Practices’ for User-Friendliness

The software we are developing is an aid for a small-scale medical research team. To borrow from the beforementioned concepts, we are developing software that helps with steps 2–5, with 3–4 essential parts of it. The focus is deliberately limited, it does not want to do more: it is simply a framework by which a team can gather all their research data, test various hypotheses and models with it, and report it automatically in a user-readable form. We feel that this scope fits with a few developers and new functions may be added as needed; in contrast with large software frameworks, where such incorporation may take a long time or workarounds.

However, the CRISP-DM standard deliberately does not provide a guideline, how to actually implement the functionality required. This problem is as old as software development itself. Fortunately, industry ‘best practices’ are available. One of these is the widely popular ‘Agile’ methodology [6]. The ‘Agile Manifesto’ of software developers describes essential concepts and introduced the concept of ‘user stories’ [7]. With Agile and with ‘user stories’, the system is approached from outside, from the perspective of the users. Everything else is developed around the needs of the users, every user has a ‘story’ with the software piece.

In a small research project, we usually have four types of participants:

• Assistants, who usually have a narrow, well-defined scope of work.
• Junior researchers (e.g., undergraduate students) and other personnel, who are strongly dependent on input in their work.
• Senior researchers (e.g., postdocs), who are able to work independently with minimal input.
• Principal investigator, who oversees the whole project and sets goals.

To meet the requirements of data processing in such teams, we have devised a software toolkit, a data learning framework we named ‘PyTanito’. The Py stands for ‘Python’, an increasingly popular, open-source, machine programming language [8], with a large following in the science field as of 2021. We have deliberately made this choice, so that our software may be extended by future researchers without ‘strings attached’ or without potential large costs that come with proprietary systems managed by large corporations. A secondary benefit of this adoption is that an increasingly large amount of ‘libraries’ are available for it. These ‘libraries’ are pieces of independent programs performing specific tasks so that one can rely on the work of others. We do not need to program every function, especially data analysis or machine learning tasks, which are usually difficult to code.

Illustrated in Figure 2, the system builds around the needs of four types of users. The respective four ‘user stories’ are as follows, roughly corresponding to the natural composition of a small medical research team:

• Software/database manager, who manages the software; may not need to be a research person; sets up environment and checks database and software function.
• Data curator, the role of assistants and junior researchers; collects and inputs data into the system.
• Simple data analyser, the role of more senior researchers; may perform simple analyses on certain selected data.
• Complex model builder and analysis, the role of the most senior researchers; sets up different machine learning models.

‘PyTanito’ incorporates another software development ‘best practice’: the layers only interact with the single layer below their level—or, at maximum, at their level. This key part is illustrated in Figure 2, which shows the direction of information flow. For example, it is not possible and not desirable for any user to interact directly with the database sitting in the centre. Simply use the 2nd role functions that read and input data into it. Another advantage is that 3rd and 4th role users do not need to know where the data comes from, as they work with the database, where all data is unified.

2.2. Data Storage and Organisation—An Up-to-Date Solution Using a Graph Database

At the core of the system is a database. It is a central, ‘single point of truth’ type organisation so that all data we work with will be consistent. We do not have to worry about differently formatted spreadsheet files or other nonstandard sources. We have chosen a graph-type database [9], instead of a traditional, relational type (such as SQL types, like Oracle SQL or MySQL [10]). Traditional relational type databases are organised into columns and rows, and usually need to have clearly defined data types/fields/tables [11]. That is, we need to have a clear concept beforehand of what we will store. Unfortunately, we found out that our exploratory research is not kind to this type of organisation. New variables emerge constantly as a result of research. In our case, not all parameters are measured for all patients, and new parameters are added routinely to certain patients. Patients do not have all their parameters measured—some patients have extra parameters. From a traditional relational database viewpoint, it is untidy and would require constantly changing its tables and columns, thus rebuilding the entire database structure.

A graph database does not rely on such assumptions. A graph, as a mathematical principle, consists of only ‘nodes’ and ‘links. The nodes may be connected with links to each other, describing their relation. We illustrate the use of this graph-type organisation on our dataset in Figure 3 [12], which consists of ~200 thrombosis patients along with their blood and blood clot parameters. Illustrated in Figure 3a, our minimal graph database consists of a ‘patient’ node and a ‘measurement’ node. That is, the minimal sensible unit is a single patient with a single laboratory parameter value.

Patients have an ID as a property of the node. The ID we added is a four-letter number and is anonymized; it cannot be used to reveal the identity of the patients. The laboratory measurements are of course nodes themselves. Patients are linked to their laboratory parameter(s) with a ‘measured’ relation link. The ‘measurement’ nodes have the value of the measured parameter as property, but others may be added, such as the name of the parameter, the method by which it was measured, the date, etc.). This arrangement gives extreme flexibility: patients may be added without much data, new data types may be added as simple new nodes with different labels, and this does not need database rework, just an additional operation to an already existing database. The question arises, that with such advantages, why graph databases are not more widespread? The answer is that the background processes driving graph databases are more computationally intensive than those driving SQL databases, but this is not a concern when using relatively small datasets with contemporary hardware.

Our actual implementation uses the Neo4j Community Edition database, which is open source, and was chosen on grounds discussed before. We note that many more types of graphs exist, such as hypergraphs, where links (the edges) may link to any number of nodes. The more commonly encountered ‘property graphs’ described in this paper and software are a subset of such hypergraphs since a particular link ‘only’ connects two nodes. The universality of hypergraphs makes them more difficult to comprehend and use computationally, nevertheless, see [14].

2.3. Data Analysis—Typical Tasks, Terms and Algorithms

In the next sections, now that we have data on hand, we need to specify, what we do with it [15,16]. For the sake of simplicity, we will provide examples using our actual patient dataset with blood parameters. In broad terms, we usually have three different types of analysis. Let us also have two types of variables, X and Y.

• Regression/classification analysis. We would like to predict Y from given X. X is the independent variable and Y is the dependent variable (X → Y).
• Hypothesis testing. If, e.g., given two patient populations (Y_A and Y_B), is the so-called null hypothesis true? The ‘null hypothesis’ usually is that the two populations are different (Y_A → X is not equal to Y_B → X). The ‘alternative hypothesis’ is the opposite, exclusive hypothesis: the two populations are identical.
• Clustering/dimensionality reducing algorithms. Sometimes we do not even have labelled data (here denoted Y), only raw data X. We need algorithms that work when we want to reduce the complexity somehow. In effect, can we cluster/label/group/classify the data based on X alone?

To put these analysis possibilities in context, taken our clotting disorder data [12], usually the Y dependent variable is a fundamental, clinically relevant label: what disease did our patients suffer from? Stroke, myocardial infarction or peripheral thrombosis? The Xs, the independent variables are usually the measured blood/blood clot parameters, such as cell counts, molecular marker (e.g., CRP) levels, etc. Please note that in a ‘regression analysis’, the Y is a continuous, quantifiable variable, while in a ‘classification analysis’ the Ys are discrete, non-quantifiable labels.

It is also useful to know that when we talk about ‘labelled data’, we mean data, which was appropriately tagged with useful (in the medical field this means clinically relevant) properties using discrete, non-quantifiable variables. This is, e.g., the disease type mentioned before. Unlabelled data are usually less worthy, as the number of analysis tools available are reduced. Here, using our dataset as an example, raw data would be, e.g., blood test numbers. If we add disease type and other clinically relevant labels, our data becomes labelled. This labelling task can be a hard problem, and often requires human intervention.

Supervised vs. unsupervised learning are additional terms encountered. In supervised learning, we have some expectations about the data, we have labels on the data, and we direct the analysis using this expectation. The analysis tools mentioned before Types 1 and 2 are supervised types. Type 3 is of the unsupervised type.

We provide two tables: one lists the common machine learning algorithms

Table 1. Machine learning algorithms. This table provides an overview of algorithms that may be used in a Python language environment. The list may be extended significantly as new algorithms emerge and get incorporated into the Python environment. Of course, discussions on these methods fall beyond the scope of this paper, so the interested readers are directed to the excellent books of Hastie and Landau [15,16] for more information.

Name of Method	Recommended Use	Method Type	Input Variables	Output	Reference to Python Implementation
Decision tree	To visusalize and break down complex decision making processes. It also provides an explainable output.	Supervised learning (in its basic form it is used for classification, but can be extended for regression)	Discrete and continuous data	Trained model (tree, in the case of CART it is a binary tree), cllassified points, (predicted values in the case of regression)	From sklearn.tree import DecisionTreeClassifier, from sklearn.tree import DecisionTreeRegressor
Random forest	It performs well on large datasets and reduces overfitting compared to decision trees. Random Forest is faster than Bagging	Supervised learning (both Classification and Regression)	Discrete and continuous data	trained model (trees for decision making) and classified points or predicted values (in the case of regression)	From sklearn.ensemble import RandomForestClassifier, from sklearn.ensemble import RandomForestRegressor
SVM (support vector machine)	It is often used for text classification tasks such as category assignment, image recognition, and performs especially well in aspect-based recognition. It can be used in linearly separable and non-separable cases as well.	Supervised learning (both Classification and Regression) In basic form it supports binary classification, but it can be extended for multiclass classification problems and for regression (support vector regression, SVR) as well.	Discrete and continuous data	Trained model, classified points or predicted values (in the case of regression)	From sklearn.svm import SVC
KNN (K-nearest neighbour)	It is a very simple algorithm, which performs much better if all of the data have the same scale, and works well with a small number of input variables, but struggles when the number of inputs is very large. It is also needed, that the training is continuous and it only works when new data is coming.	Supervised learning (classification)	Discrete and continuous data	Classified points or predicted values (in the case of regression)	From sklearn.neighbors import KNeighborsClassifier
Linear Regression	One of the most frequently used regression methods, when a linear relationship is assumed between the input and output variables.	Supervised learning (Regression)	Continuous data	Regression line (predicted points)	From sklearn.neighbors import KNeighborsClassifier
Logistic regression	It is a commonly used algorithm for solving binary classification problems.	Supervised learning (Binary Classification with Softmax extension it can solve multiclass classification problems)	Discrete and continuous data	Classified points-binary (discrete) value	From sklearn.linear_model import LogisticRegression
ANN (artificial neural network)	State of the art. It performs well on a large and multi-dimensional dataset, and should be applied when there is no explicit mathematical equation for solving the problem. it is especially useful in image analysis, text classification, voice processing tasks. The results are not explainable.	Supervised learning (both classification and regression). It can be used also for unsupervised learning tasks, but it is not so widespread.	Discrete and continuous data	Trained model, classified points or predicted values (in the case of regression)	From sklearn.neural_network import MLPClassifier, from sklearn.neural_network import MLPRegressor
k-means	Clustering is used when we want to divide our datapoints into groups according to smilarity. The k-means algorithm can find spherical clusters easily, while DBSCAN can find clusters of any shape as long as the dataset has balanced density. Moreover, there are algorithms which are also good at the clustering of datasets with unbalanced density, such as mean-shift.	Unsupervised learning (clustering)	Discrete and Continuous data	Trained model, clusters	From sklearn.cluster import KMeans
DBSCAN					From sklearn.cluster import DBSCAN
Gaussian mixture model					From sklearn.mixture import GaussianMixture
Mean-shift					From sklearn.cluster import MeanShift
Others					From sklearn.cluster import AffinityPropagation, from sklearn.cluster import AgglomerativeClustering, from sklearn.cluster import Birch
PCA (principal component analysis)	Dimensionality reduction can serve as data preparation, as the machine learning algortihms work better on low-dimension data. However, the reduction can be the main exercise too.	Unsupervised learning (dimensionality reduction)	Discrete and continuous data	Data with reduced dimensionality (less features), where as much information as possible about the original data is preserved.	From sklearn.decomposition import PCA
LDA (linear discriminant analysis)					From sklearn.discriminant_analysis import LinearDiscriminantAnalysis
Others					From sklearn.decomposition import TruncatedSVD, from sklearn.manifold import Isomap, from sklearn.manifold import LocallyLinearEmbedding
Association	One uses Association when the aim is to study the connection between the datapoints, and make recommendation for new datapoints.	Unsupervised learning (association)	Discrete and continuous data	Association rules, associations	From mlxtend.frequent_patterns import apriori, association_rules
T-test	To compare the means of 2 groups. It assumes that the samples are (approximately) normally distributed and are independent, and have a similar amount of variance within each group.	Hypothesis testing	Continuous data, normally distributed, small sample with unknown variance	t-value, p-value, degrees of freedom	From scipy.stats import ttest_ind
F-test	The hypothesis that the means of a given set of normally distributed populations, all having the same standard deviation, are equal. The hypothesis that a proposed regression model fits the data well. (lack-of-fit sum of squares). The hypothesis that a data set in a regression analysis follows the simpler of two proposed linear models that are nested within each other.	Hypothesis testing	Independent and normally distributed data with a common variance.	F-value, p-value, degrees of freedom	From scipy.stats import f_oneway
Z-test	One can use Z-test when the sample size is greater than 30, the data points are independent from each other, the data is (approximately) normally distributed, sample sizes are equal. To check whethet sample mean of the two groups are equal or not.	Hypothesis testing	Independent and normally distributed data with large (>50) sample size or the variance is known	Z-value, p-value, degrees of freedom	From statsmodels.stats.weightstats import ztest
Chi-cquare Test	Chi-squared test is used to determine whether there is a statistically significant difference between the expected frequencies and the observed frequencies in one or more categories.	Hypothesis testing	Random sample that are classified into k mutually exclusive classes	p-value	From scipy.stats import chisquare

. The second table lists the evaluation metrics that may be used to determine how successful we were by using the beforementioned algorithms

Table 2. Metrics used for evaluation of machine learning models. In this table, we provide a list of useful metrics used when quantifying success in a data analysis experiment, be it a regression or classification problem. As with Table 1, the list is not exhaustive, and interested users are directed to [17,18,19] for more information.

Name of Error Metrics	Recommended Use	Typical Drawback	Input Variable	Output Type and Range	Reference to Python Implementation
Mean absolute error/MAE	Regression. Gives an absolute measure; can be used to compare regression models on the same dataset.	Specific for the given model and dataset.	Continuous	Continuous value, [0; ∞]	From sklearn.metrics import mean_absolute_error
Mean square error/MSE					From sklearn.metrics import mean_squared_error
Root mean square error/RMSE					From sklearn.metrics import mean_squared_error
Standard error/residual standard error					From scipy.stats import sem
R2-value/R squared	Regression. A basic metric to evaluate regression models.	Does not take into account the number of independent variables.	Continuous	Continuous value, [0; 1]	From sklearn.metrics import r2_score
Adjusted R2/adjusted R square	Regression. Useful to compare models with differing numbers of independent variables.		Continuous	Continuous value, [0; 1]	Import statsmodels.api
Confusion matrix	Classification. Can be used to calculate other classification metrics.	The simple quantities of true positives (TP)/true negatives (TN)/false positives (FP)/false negatives (FN) refer only to the given model and dataset.	Discrete	Matrix for each classes values [0; ∞]	From sklearn.metrics import confusion_matrix
True positive rate/TPR/recall/sensitivity/probability of detection	Classification. Recommended when the costs of FN is high (such as sick patient detection).	Not recommeded to use alone.	Discrete	Continuous value, [0; 1]	From sklearn.metrics import recall_score
True negative rate/TNR/specificity	Classification				From sklearn.metrics import confusion_matrix
False positive rate/FPR/type I error	Classification				From sklearn.metrics import confusion_matrix
False negative rate/FNR/type II error	Classification				From sklearn.metrics import confusion_matrix
Accuracy	Classification. Useful to compare classification models on the same dataset. Recommended when every class is equally important.	Can be largely contributed by a large number of TN. Not recommended when the costs of having a mis-classified actual positive is high (ex. virus carrier). Not recommeded to use alone.	Discrete	Continuous value, [0; 1]	From sklearn.metrics import accuracy_score
Misclassification rate/classification error	Classification. Since it is = 1-Accuracy, all properties are the same
Precision	Classification. Recommended when the costs of FP is high (such as email spam detection).	Not recommeded to use alone.	Discrete	Continuous value, [0; 1]	From sklearn.metrics import precision_score
F1 score/F-score/F-measure	Classification. To have a balance between Precision and Recall and there is an uneven class distribution (large number of actual negatives)		Discrete	Continuous value, [0; 1]	From sklearn.metrics import f1_score
F2 score	Classification. The same as F1 score, but we nominate twice importance to recall over precision.				From sklearn.metrics import fbeta_score
Micro F1, macro F1, weighted F1	Multi-class classification. Advanced versions of F1 score where the difference (micro, macro, weighted) is the type of averaging performed on the data. Micro precision, micro recall weighted precision, macro precision, etc., also exist.		Discrete	Continuous value, [0; 1]	From sklearn.metrics import f1_score
ROC (receiver operating characteristics)	Classification. Visualizes the tradeoff between TPR and FPR.	In some cases, can be difficult to compare more curves *2	Discrete	Curve, X axis = FPR, Y axis = TPR	From sklearn.metrics import roc_curve
ROC AUC (area under ROC)	Classification. A quantity to describe the ROC curve.	Not recommended when the data is heavily imbalanced *3	Discrete	Continuous value, [0; 1]	From sklearn.metrics import roc_auc_score
PR curve (precision–recall curve)	Classification. Combines precision and recall in a single visualization.		Discrete	Curve, X axis = Recall, Y axis = Precision	From scikitplot.metrics import plot_precision_recall
PR AUC (area under PR curve)	Classification. A quantity to describe the PR curve.		Discrete	Continuous value, [0; 1]	From sklearn.metrics import average_precision_score

.

2.4. A Machine Learning Workflow—Common Processes Explained with Examples

In a traditional scientific project, we are convinced of our models and hypotheses when these are repeated and verified by independent researchers. A single team collects data, and then ‘trains’ their model/algorithm on a part (or whole) of the available data [16]. Thus, this data is called ‘training data’. Training a model/algorithm means the setting of the model’s parameters so that the model from Table 1 describes the data ‘best’, according to selected metrics seen in Table 2. Training often involves other kinds of parameters (hidden variables) during the construction of the model (‘hyperparameters’) that refine/restrict the behaviour of an otherwise complex model. The model’s ordinary parameters are, e.g., the two coefficients of a simple linear regression, and in this case, the training may be called ‘linear fitting’. Linear regression is simple, and it has not hidden ‘hyperparameters’, but more complex models, such as a ‘decision tree’ may have more parameters and several hyperparameters that describe it.

However, it is possible that the model is not optimal for the problem, for example, a linear regression does not capture the phenomena studied, and so does not make sense from a medical/physical/chemical standpoint. To see that the fitted model is reasonable (and does not depend on cherry-picked ‘training’ datapoints), it is good practice to have a ‘validation set’ of data. This ‘validation dataset’ is data totally separated from the ‘training data’. It is not used for the training (e.g., linear regression fitting), and so is ‘unseen’ by the model-building procedure. The trained model’s error on this ‘validation dataset’ is more indicative of the model’s appropriateness.

However, even this train-validation procedure may be less than optimal in real life. For example, because of inherent systematic errors during the experiments or if it just happens that the particular picking of data into train/validation sets skew results. Figure 4 illustrates how independent observations by different researchers solve the problem. However, in the medical field, this is often not possible, because the experiments may be too costly or there is simply not enough interest or resources to reproduce results.

A so-called cross-validation procedure Figure 4b may solve this problem with caveats [20,21]. In a cross-validation procedure, we produce simulated ‘independent’ datasets randomly. A k-fold cross-validation procedure produces ‘k’ number of simulated datasets. This procedure minimizes at least the ‘cherrypicking’ problem of the train-validation dataset split. Still, the problem of systematic errors remains, therefore, this procedure produces only a hint on the minimum error, and the real error is certainly larger than that. Still, the procedure will prevent over-optimistic error estimations by setting a minimum error expectation and is best used to compare the appropriateness of different machine learning model types (e.g., linear regression vs. polynomial regression, etc.). Another advantage is that this procedure may be used ‘on top’ of other error metrics, because any error metrics may be used with these ‘independent’ datasets.

In the last part of this paper, we show the actual outputs of the software. Figure 5 depicts the use of the software to check the data and available measurements for further analysis (a simple ‘data curator’ user role). Figure 5 displays the command-line interface which is always available for Python programming. The output is also always available as a command-line type. Although not as wieldy or ‘nice’ as graphical user interfaces (GUIs), command-line interfaces and outputs serve an important purpose: they are read and parsed easily by machines, e.g., by other parts of the same program or even third-party software. For this reason, efficient software implements both the command-line interface and the GUI.

In Figure 6, we show an actual example of decision making with the help of machine learning. The figure shows a pair of plots for two different, but related prediction problems using common ‘Decision Tree’ algorithm implementation. In each of the plots, we compare several ‘tree’ models and decide which ‘tree’ is the best if there is such a model. The provided example is realistic in the sense that it also displays commonly encountered, relatively unsuccessful models. A major takeaway is that relying on a single metric on a single training run is guaranteed to be flawed: even when it seems to be better, it is not when many random combinations of data are also validated. Extensive use of cross-validation helped in our case to set realistic expectations. For this particular example, the conclusion is that the decision trees are better than random guesses, but not by very much. It is an open question as to whether robust, clinically relevant predictions can be made. More data, more types of variables and more types of algorithms need to be tested. Thus, from this data only, the prediction of disease before onset is not practical.

3. Conclusions and Remarks

We have seen that modern workflow (CRISP-DM standard) with modern software ‘best practices’ (‘Agile’ and software layering) can produce useful tools in medical research. We also provide a glimpse into how data may be stored in a humanly reasonable way using a graph database. We also provided lists of the incredible amount of contemporary machine learning methods and metrics for their evaluation.

We consider two kinds of readers and have conclusions for each.

• First, from the perspective of the software developer involved in a ‘decision making’ project, it is notable that software specification is a bottleneck during the initial steps. This often stems from a lack of clarity and vision during the initial stages of the project. Going forward in the project, the tuning of the machine learning models (‘hyperparameter tuning’) becomes the bottleneck and this remains so until the end. Curiously, the choice of algorithms does not appear critical now. Computational capacities are larger than ever, and several highly advanced machine learning methods are available (in fact, often more than needed in a project). Projects, however, may fail easily because of data quality issues, and this brings us to the second set of conclusions below.
• Second, from the perspective of the medical professional, the bottleneck is usually a lack of understanding between them and software specialists during the initial stages of the project. Going forward, the single but often critical bottleneck is the quality of their data. Experiments may span years and may be performed by different personnel over time. This may lead to inconsistent procedures, even if for mundane reasons, e.g., because of no longer available reagents or altered cell lines. These can introduce large systematic errors that make even advanced machine learning algorithms ineffective. Other kinds of experiments are inherently difficult to perform consistently for physical, economical or legal reasons. For example, in our case, the blood clots of ex vivo human origin pose a significant problem, since the number of experiments is severely limited. In summary, medical professionals must do everything to ensure reproducibility and consistency of their experiments and data over long timespans in sufficient quantity.