Important on OPTO norms
Each norm group has a norm for each of the 20 Aspects in OPTO. To ensure that the norms are representative of the local population, the norms are weighted to reflect the population distribution in terms of gender, age, and educational level.
As example: Given that the Norwegian data sample for creating norms has 59% male, and 41% female, and the actual population distribution of gender in Norway is 50% male and 50% female, than the male and female statistics are given a weight of .50 each in the final norm, ensuring that the norm is representative of the population.
In the international norm, the different local / national norms are also given equal weight in terms of sample size. What this means, is that the Norwegian norm group (N=5893) and the Mexican norm group (N=530) are given equal weight in the international norm.
You can read more on OPTO norms by this link