How iWise differs from SM Marks in calculating combined marks

Although not strictly an instruction, this chapter has been included because a few schools have asked how iWise calculates the mark for a combined task and why this can differ from what appears to be the same combined tasks in SM Marks.

The two main differences are:

1.	When scaling using a method that requires a standard deviation, iWise will use population, not sample standard deviation.

This is a more appropriate STDev method to use as we have every data point in the set.

It is also used by the Board of Studies. http://www.documentingexcellence.com/stat_tool/SD.htm

2.	When combining tasks together, iWise will pre-scale the components to the desired target scaled parameters, then combine, and then rescale the combined result to the desired scaling parameters. SM Marks doesn’t operate in exactly the same way.

This approach will also lead to a different result when compared to just adding the results together, and then applying the scaling method to the resulting marks. Pre-scaling the components to the desired scaling parameters can remove some weighting bias in the original tasks.