This group of graphs demonstrates the bimodality of ET parameters. Each graph is a box-plot presentation of "good" and "bad" populations. On the Y axis are values of the ET parameter, along the X axis are the wafer numbers. The box-plot can tell you a lot about the distribution of ET parameter per wafer. Each box demonstrates the distribution of ET values measured on wafers with the same number, taken from historical data.

The floor and ceiling of the box locate the ET parameter's 25th and 75th percentiles, respectively, on the Y axis. The height of the box is equal to the interquartile range, computed by subtracting the 25th percentile from the 75 percentile. The horizontal line inside the box identifies the median. Each outlier is identified by an asterisk.

On these graphs you can see that the behavior of "good" and "bad" populations clearly differs. Note also the statistical characteristics of these populations -- they are almost the same.