The first rule of data organization is be consistent.

• Use consistent codes for categorical variables. For a categorical variable like sex, use a single common value for males (e.g. “male”) and a single common value for females (e.g. “female”). Don’t sometimes write “M”, sometimes “male”, and sometimes “Male”. Pick one and stick to it.

• Use a single fixed code for any missing values. I prefer to have every cell filled in (more discussion here), so that one can distinguish between truly missing values and unintentionally missing values. R users prefer “NA”. You could also use a hyphen. But stick with a single value throughout your data. Definitely don’t use a numeric value like -999 or 999; it’s easy to miss that it’s intended to be missing. Also, don’t insert a note in place of the data, explaining why it’s missing. Rather, make a separate column with such notes.

• Use consistent variable names. If in one file (say the first batch of subjects), you have a variable called “Glucose_10wk”, then call it exactly that in other files (say for other batches of subjects). If it’s variably called “Glucose_10wk”, “gluc_10weeks”, and “10 week glucose”, then downstream the data analyst will have to work out that these are all really the same thing. (More on naming variables here.)

• Use consistent subject IDs. If sometimes it’s “153” and sometimes “mouse153” and sometimes “mouse-153F” and sometimes “Mouse153”, there’s going to be extra work to figure out who’s who.

• Use a common data layout in multiple files. If your data are in multiple files, use the same layout in all files. (More on layout here.)

• Use consistent file names. Have some system for naming files. If one file is called “Serum_batch1_2015-01-30.csv”, then don’t call the file for the next batch “batch2_serum_52915.csv” but rather use “Serum_batch2_2015-05-29.csv”. (More on naming files here.)

• Use a single common format for all dates, preferably YYYY-MM-DD, like 2015-08-01. If sometimes you write 8/1/2015 and sometimes 8-1-15, you’re asking for trouble. (More on dates next.)

• Use consistent phrases in your notes. If you have a separate column of notes (for example, “dead” or “lo off curve”), be consistent in what you write. Don’t sometimes write “dead” and sometimes “Dead”, or sometimes “lo off curve” and sometimes “off curve lo”.

• Be careful about extra spaces within cells. A blank cell is different than a cell that contains a single space. And “male” is different from “ male ” (that is, with spaces at the beginning and end). These can be a headache later on.

Next up: Write dates as YYYY-MM-DD.