Update README.md

README.md

@@ -9,3 +9,6 @@ All code is packaged in a python module called `stat159lambda`.
 Ensure this module is on your python path with `export PYTHONPATH='<path-to-repository>/code'`.  
 For example, `export PYTHONPATH='/Users/alondaks/project-lambda/code'`.
 
+## Environment Variables
+Data for a single subject is ~7.5 GBs due to 2 hour scan at 7 Tesla resolution. The preprocessing scripts are therefore memory intensive. Reproducing preprocessing code should be on a machine with 70+ GBs. In addition to downloading raw data, ``make data`` from the root project directory will download all preprocessed data. Setting a unix environment variable ``USE_CACHED_DATA`` will instruct scripts if they reexecute preprocessing or not. ``USE_CACHED_DATA='True'`` will bypass any presprocessing as long as the resulting preprocessed file exists in ``data/processed/``. ``USE_CACHED_DATA='False'`` will recalculate preprocessed files even if they exist in ``data/processed/``. ``USE_CACHED_DATA`` will default ``'True'``.
+