A Participant in MCPS is a resident of one of two districts in Mexico City who was visited in their home and consented to the collection of information about them.

The participants property of a Data-Field is the number of MCPS participants on whom that particular item of data is held. Except for baseline data-fields, this number will generally be less than the total number of participants in MCPS since, for various reasons, not everyone took part in every investigation or measurement.