I am an ArcGIS 10.2.1 user. Recently, I used summary statistics tool to obtain mean values for each unique `water quality parameter` measured on different days at various monitoring stations. The input table is fairly large, it has approximately 2 million rows. I used the field that has the names of the `water quality parameters` as case field in order to obtain the mean for each unique parameter. When I checked the output table, I noticed that some parameters were duplicated. Having observed this unexpected result, I decided to use frequency tool to check whether a similar duplication would also occur with this tool or not. The frequency output table also included duplicate `water quality parameter` rows. I attach the screen capture of the first output table delivered by `summary statistics` tool to this post.