human
1. Pre-registering a Test Statistic
The hypothesis we will test in this tutorial is the model outlined in the next section - but how well that model fits the data is a question we will answer in part using a test statistic. 
Having understood what the data represent (and had a quick look at them), what feature in the data do you want your model to explain well?  
With this in mind, what is a good test statistic to summarize the data? Spend a few mins thinking about this and discussing it, then implement is before moving on. You'll then use this "pre-registered" test statistic in a Bayesian model check below.
Your test statistic should be a function of the data only, although in general it's also possible to use statistics that are functions of both the data and model."
Please write code following the instructions in jupyter notebook style.
assistant
#Here is code to 1.

try:
    exec(open('solutions/teststatistic.py').read())
except IOError:
    REMOVE_THIS_LINE()
    def T(yy):
        """
        Argument: a data vector (either the real data or a simulated data set)
        Returns: a scalar test statistic computed from the argument
        """
        REPLACE_WITH_YOUR_SOLUTION()