“Good news, everyone!” is a collection of articles about solving problems that a data scientist might encounter in practice. At least, this is the intent; we will see how it goes.

## Articles

### Sample size determination using historical data and simulation

In order to test a hypothesis, one has to design and execute an adequate experiment. Typically, it is neither feasible nor desirable to involve the whole population. Instead, a relatively small subset of the population is studied, and given the outcome for this small sample, relevant conclusions are drawn with respect to the population. An important question to answer is then, What is the minimal sample size needed for the experiment to succeed? In what follows, we answer this question using solely historical data and computer simulation, without invoking any classical statistical procedures.

### A Bayesian approach to the inference of the net promoter score

The net promoter score is a widely adopted metric for gauging customers’ satisfaction with a product. The popularity of the score is arguably attributed to the simplicity of measurement and the intuitiveness of interpretation. Moreover, it is claimed to be correlated with revenue growth, which, ignoring causality, makes it even more appealing. In this article, we leverage Bayesian statistics in order to infer the net promoter score for an arbitrary segmentation of a customer base. The outcome of the inference is a distribution over all possible values of the score weighted by probabilities, which provides exhaustive information for the subsequent decision-making.

### Interactive notebooks in tightly sealed disposable containers

It is truly amazing how interactive notebooks—where a narrative in a spoken language is entwined with executable chunks of code in a programming language—have revolutionized the way we work with data and document our thought processes and findings for others and, equally importantly, for our future selves. They are ubiquitous and taken for granted. It is hard to imagine where data enthusiasts would be without them. Most likely, we would be spending too much time staring at a terminal window, anxiously re-running scripts from start to finish, printing variables, and saving lots of files with tables and graphs on disk for further inspection. Interactive notebooks are an essential tool in the data scientist’s toolbox, and in this article, we are going to make them readily available for our use with our favorite packages installed and preferences set up, no matter where we find ourselves working and regardless of the mess we might have left behind during the previous session.

### On the expected utility in conversion rate optimization

It can be not only extremely useful but also deeply satisfying to occasionally dust off one’s math skills. In this article, we approach the classical problem of conversion rate optimization—which is frequently faced by companies operating online—and derive the expected utility of switching from variant A to variant B under some modeling assumptions. This information can subsequently be utilized in order to support the corresponding decision-making process.

### A poor man’s orchestration of predictive models, or do it yourself

As a data scientist focusing on developing data products, you naturally want your work to reach its target audience. Suppose, however, that your company does not have a dedicated engineering team for productizing data-science code. One solution is to seek help in other teams, which are surely busy with their own endeavors, and spend months waiting. Alternatively, you could take the initiative and do it yourself. In this article, we take the initiative and schedule the training and application phases of a predictive model using Apache Airflow, Google Compute Engine, and Docker.