There are a number of research design methods that are employed to study human development. Briefly define the following types of research design methods: cross-sectional, longitudinal, and sequential designs. Give one example of a weakness of cross-sectional and longitudinal designs and explain how a sequential design attempts to address these limitations.

A cross-sectional study compares groups of individuals of different ages at the same time. A weakness of cross-sectional studies is the difficulty of obtaining equivalent samples at different ages.
A longitudinal study follows a single group of individuals as they develop. A weakness of longitudinal studies is that they necessarily take years, and not everyone who participates the first time is willing and available later. Furthermore, those who remain in a study may differ in important ways from those who leave.
A sequential (or "cross-sequential") design combines the advantages of both cross-sectional and longitudinal designs. In a sequential design, researchers start with groups of people of different ages, studied at the same time, and then study them again at one or more later times.