Math A: §8.3 measures of spread
In the next slide they talk about: 'mean deviation from the mean'
(if you know what I mean...).
This might seem difficult, so I translate this, for a change:
'de gemiddelde afwijking van het gemiddelde'.
To calculate this, you first calculate the mean.
How to go about further, see next slide (this text is on p.19 too!).