1. data or information obtained prior to or at the onset of a study (e.g., before introduction of an intervention) that serves as a basis for comparison with data collected at a later point in time so as to assess the effects of particular manipulations or treatments. For example, a memory researcher may measure how many words from a list participants remember initially and then compare that figure to the number of words they remember following the use of a new mnemonic technique (i.e., the experimental manipulation).

2. more generally, any stable level of performance used as a yardstick to assess the effects of particular manipulations or changes.