When n=1, use the I-MR control chart but in that chart sigma calculated used by d2 and when we are select the d2 value as per sample size so why in that case we use d2=1.128 means d2 value of sample size 2.
I do agree that 2 data points required for moving range chart, but those two observations or data points belong to same group then how can we consider them as 2 groups & d2 is taken as 1.128 which is applicable for 2 groups. Please give clarity.
The MR = |previous-current| This is 2 data points, therefore d2 = 1.128. The reason it is a MOVING range (vs. just a range) is that the same data point is used twice (once as current and once as previous) throughout the time series. The MR is a surrogate for an actual range based on a subgroup size.
Sorry, I'll add...the MR, I chart is used when: 1. Scarcity of data (e.g., data from an experiment, summarized data at the top layer of a directed sampling plan) 2. There is no rational subgroup identified The MR can be quite useful to examine data for consistency and identify potential outliers. If, however, you are interested in comparing and assigning different sources of variation (which require rational subgroups based on rational hypotheses), you are better off with R, X-bar charts.