Normal distribution

Many measurements cluster in the middle and become rarer toward both ends at the same rate. This bell shape is called the normal distribution.

Example: The concentration scores of the "yes" group look roughly like this. In the "no" group, one very low value (9 points) disturbs the picture.

Caution: For the t test, what matters most is that the means behave roughly normally. What decides is clear skewness and outliers, not a fixed number of people: without either, Welch's test holds even with small groups.