Data for analysis are typically stored in a rectangular format, where each row represents an observation (e.g., individual level data) and each column represents a data field (e.g., age, weight, income, etc.). For longitudinal or repeated measures data (e.g., data where individuals are measured annually), each row should correspond to a measurement of an observational unit corresponding to some time point (e.g., data for individual X in year Y). Data files should also be accompanied by a “data dictionary” that describes each variable (data type, unit of measurement, possible values, missing data indicator, etc.).
To facilitate the data analysis process, we request that each data set be stored as a separate comma-separated values (CSV) text file. To export a single worksheet from a MS Excel file (*.xls, *.xlsx) to a CSV file in MS Excel, follow these instructions; please google if you use a different software or a different version of MS Excel. For example, this Data.xls MS Excel file contains two sheets. The first sheet contains demographics data and the second sheet contains annual follow-up data for each subject. We save both sheets as CSV files using MS Excel: demographics.csv and longitudinal.csv. We also accompany a data dictionary to describe the data.