Dr. Irwin is testing the same individuals repeatedly over time as part of a research study, while Dr. Jenner is comparing the performance of different people of various ages at the same time. Dr. Irwin is using a research design; Dr. Jenner is using a design.

longitudinal; cross-sectional

Explanation:

In a longitudinal study, researchers make a variety of studies of the same subjects over a period of time, often lasting several years. A longitudinal study is a research design that requires repeated measurements of the same variables over short or long periods of time. It is often a form of observational study, although it can also be classified as longitudinal randomized trials.

 cross-sectional design is a type of observational analysis that studies data from a population, or a representative subset, at a particular point in time.

Dr. Irwin is testing the same individuals repeatedly over time as part of a research study, while Dr. Jenner is comparing the performance of different people of various ages at the same time. Dr. Irwin is using a LONGITUDINAL research design; Dr. Jenner is using a CROSS-SECTIONAL design.

longitudinal; cross-sectional

Explanation:A cross-sectional study

design is one that compares different population groups at once. cross-sectional study design is that it allows a researcher to compare and contrast many variables at once. longitudinal research design unlike the former makes use of several observations over a long period of timeWhere a researcher does not interfere with his subjects.

This type of research design enables a researcher to easily detect developments on characteristics of the subject target whether group or individual Cross-sectional research take less time than longitudinal research . Researchers use cross-sectional sresearch for the establishment of target variable while a longitudinal reasearch for cause and effect.