Chapter Thirteen - Additional Resources

Box 13.1 : Independent and dependent variables

Source: Kgaile and Morrison (2006)

Box 13.2 : The effects of randomization

Select twenty cards from a pack, ten red and ten black. Shuffle and deal into two ten-card piles. Now count the number of red cards and black cards in either pile and record the results. Repeat the whole sequence many times, recording the results each time.

You will soon convince yourself that the most likely distribution of reds and blacks in a pile is five in each: the next most likely, six red (or black) and four black (or red); and so on. You will be lucky (or unlucky for the purposes of the demonstration!) to achieve one pile of red and the other entirely of black cards. The probability of this happening is 1 in 92,378! On the other hand, the probability of obtaining a ‘mix’ not more than 6 of one colour and 4 of the other is about 82 in 100.

If you now imagine the red cards to stand for the ‘better’ ten children and the black cards for the ‘poorer’ ten children in a class of twenty, you will conclude that the operation of the laws of chance alone will almost probably give you close equivalent ‘mixes’ of ‘better’ and ‘poorer’ children in the experimental and control groups.

Source : Adapted from Pilliner, 1973

Box 13.3 : Interaction effects in an experiment