Upper-lower groups are behind Lertap's Stats-ul and ECStats-ul summaries.  They're formed by first sorting all student test scores from highest to lowest, and then, in the case of 5 groups ("quintiles"), marking the top 20% of the students, those with the best scores, as the "top" group, taking the next 20% as the 2nd-best group, the next 10% will be the 3rd group, the next 20% the 4th, and finally, the bottom 20%, the lowest group, ending up with five levels of proficiency.

 

The top group is referred to in some Lertap documentation as the "upper" group, with the lowest 20% called the "lower" group.  In 2015 group names were changed -- the "upper" group became "Grp1", and, in the case of quintiles, the "lower" mob became "Grp5".

 

The number of upper-lower groups is set in row 13 of the System worksheet.

 

Prior to 2015, the maximum number of upper-lower groups was set at 5; this has been changed to 10.

 

When Elmillon starts to run, it makes a check on group sizes: it divides the number of students by the number of upper-lower groups set in the System worksheet.  If the result is a number less than 20, and if the "Number of upper-lower groups adjuster" option is set to "Yes" (row 91 of the System worksheet), then users are asked if they'd like to "reduce the number of upper-lower groups for this run".

 

The only reason this question is posed has to do with the plots produced by the "Res. charts" option when users are working with a Stats-ul or ECStats-ul summary.  These plots can look "jerky" when there's a small number of students in the upper-lower groups and, in these cases, reducing the number of groups can (at times) smooth out the plots, at least a bit.

 

See this pdf document for more information.