Statistics: central tendency, dispersion, NPC, tests

Statistics turns raw scores into decisions: central tendency locates the centre, dispersion shows the spread, the normal curve gives a reference shape, and tests judge whether a difference is worth believing. For example, five scores 2, 4, 6, 8, 10 have mean, median and mode all equal to 6 only because the distribution is symmetric — change one score and they part company. This chapter works through each tool with numbers.