|This article describes the weighting algorithm used in AskiaAnalyse
|weighting; weight; algorithm; analyse; askiaanalyse
Documentation Note : merge with http://analysishelp.askia.com/weighting_analyse
The process of weighting involves changing the weight of the interviews to correct sampling errors.
Here is an example: we interviewed 60 men and 40 women. The population we want to describe is equally composed of men and women. To correct this, each time we run counts on a question, instead of adding 1 for each man, we will add 50/60 = 0.8333, and for each woman we will add 1.25. In other words, the under represented categories will have a stronger vote than the over represented ones.
The process gets more complicated when more than one variable is involved. Let us imagine we want to apply a weighting on gender and social grade (A,B,C1). If we have the information on how the gender is defined for each social grade (and hence creating a new variable with 6 cells), we can fall back on the one variable weighting. But if we increase the number of questions and responses in the weighting, the target population frequency in each of the sub-cells may not be known.
Another alternative would be to assume that gender and social grade are independent and therefore we should observe on each social grade a 50-50 split on gender. This hypothesis is very restrictive and would very likely skew the results. Therefore the algorithm is an iterative process.
- First the weights are set to 1 for all interviews.
- The system finds which variable is the furthest from the targets.
- The current weights are changed so this variable will exactly fit the targets.
- The minimum and maximum weights are applied if they are defined.
- If all variables fit the targets then the process will stop otherwise it will start again at b).
After a few iterations the weights should converge to an acceptable solution.