Section author: Danielle J. Navarro and David R. Foxcroft

Descriptive statistics separately for each group

It is very commonly the case that you find yourself needing to look at descriptive statistics broken down by some grouping variable nominal. This is pretty easy to do in jamovi. For instance, let’s say I want to look at the descriptive statistics for some clinical trial data, broken down separately by therapy type. This is a new data set, one that you’ve never seen before. The data is stored in the clinicaltrial data set and we’ll use it a lot in chapter Comparing several means (one-way ANOVA) (you can find a complete description of the data at the start of that chapter). Let’s load it and see what we’ve got:

Variables in |clinicaltrial|

Fig. 17 Screenshot of jamovi showing the variables stored in the clinicaltrial data set

Evidently there were three drugs: a placebo, something called “anxifree” and something called “joyzepam”, and there were 6 people administered each drug. There were 9 people treated using cognitive behavioural therapy (CBT) and 9 people who received no psychological treatment. And we can see from looking at the Descriptives of the mood.gain variable that most people did show a mood gain (mean = 0.88), though without knowing what the scale is here it’s hard to say much more than that. Still, that’s not too bad. Overall I feel that I learned something from that.

We can also go ahead and look at some other descriptive statistics, and this time separately for each type of therapy. In jamovi, check Std. deviation, Skewness and Kurtosis in the Statistics options. At the same time, transfer the therapy variable into the Split by box, and you should get something like Fig. 18.

Variables in |clinicaltrial| split by therapy type

Fig. 18 Screenshot of jamovi showing Descriptives of the variables in the clinicaltrial data set split by therapy type

What if you have multiple grouping variables nominal? Suppose you want to look at the average mood gain separately for all possible combinations of drug and therapy. It is possible to do this by adding another variable, drug, into the Split by box. Easy peasy, though sometimes if you split too much there isn’t enough data in each breakdown combination to make meaningful calculations. In this case jamovi tells you this by stating something like NaN or Inf.[1]

[1]Sometimes jamovi will also present numbers in an unusual way. If a number is very small, or very large, then jamovi switches to an exponential form for numbers. For example 6.51e-4 is the same as saying that the decimal point is moved 4 places to the left, so the actual number is 0.000651. If there is a plus sign (i.e. 6.51e+4 then the decimal point is moved to the right, i.e. 65,100.00. Usually only very small or very large numbers are expressed in this way, for example 6.51e-16, which would be quite unwieldy to write out in the normal way.