What does GER/NER not tell me about access and retention?

GER and NER provide a sense of enrolment trends through the number of students who are over and under age, across social categories and geographies. However, this data is only a first step. Once a student has been enrolled, regular attendance is what matters. GER/NER do not provide any sense of whether students have attended schools. Most schools collect data on attendance. These can be used to construct Gross Attendance Rates and Net Attendance Rates, similar to GER/NER. These can be analysed across:

1. Different time periods - say over a week, month, quarter, and for the year. Seasonal patterns can be used to explore why attendance may be low or high in a particular period. Does this coincide with festivals, or with the lack of facilities in schools? For instance, were mid day meals not provided in a particular period which coincides with low attendance?
2. Across social categories and geographies - if a trend of low attendance is observed for one social category, or in one geography, then the relevant authority can explore further as to why that might be the case.