Time Lag Effect
The (time) lag effect refers to differences in the performance of research subjects that are noticed over the duration of an experiment. This is most evident in experiments or observations that were carried out over prolonged periods of time, like a longitudinal study.
For instance, there are several participants in a longitudinal study on the long-term effects of pre-school attendance. As these participants are followed through their lives the researcher will see the participants taking different life and educational paths; finishing high school, college attendance, advanced degree programs, etc. Sometimes you may see a person who has taken a vocational path in life develop an interest in pursuing higher education, or someone who had been averse to marriage decide to marry and start a family.