Re: XmRTrend chart
It is recommended for continuos process, like chemical batchs.
The subgroup of 1 is used for the case when 1 is the rational subgroup of the sample (you shouldn't agrupate the data in a 5 points sample just because you want to use a subgroup of 5, the points must have a relationship which differ form the next group of data, so there is a within and between sample variation).