One of the challenges of conducting health research is that it is impossible, and unethical, to manipulate certain variables, such as sexual behaviors or smoking. The research design that examines differences between groups without manipulating these groups isa. experimental. b. ex post facto or quasi-experimental. c. random assignment. d. longitudinal.

b. ex post facto or quasi-experimental.

Research designs that examine differences between groups without actively manipulating any variables are known as ex post facto or quasi-experimental designs. The term "ex post facto" is Latin for "from after the fact." This type of study looks at variables as they naturally occur, without the researcher controlling or altering them. It is often used in situations where it is either impossible or unethical to manipulate variables, such as in studies examining the effects of smoking or sexual behaviors. Researchers observe existing conditions or differences between groups and then make inferences about the relationships between variables.

Extra: In research methodology, there are several types of research designs, and each serves a specific purpose and adapts to the ethical and practical constraints of studying various subjects:

1. Experimental Research: In this type of research, the researchers manipulate one or more independent variables to observe the effect on one or more dependent variables while controlling external factors. This method can establish cause-and-effect relationships but may not always be feasible or ethical, especially in health research involving risk factors.

2. Quasi-Experimental Research: As mentioned earlier, this type of research resembles experimental research but lacks random assignment of subjects to treatment and control groups. Quasi-experiments are useful when random assignment is not possible due to ethical or practical reasons, but researchers still want to explore cause-and-effect relationships.

3. Ex Post Facto Research: This is a specific subtype of quasi-experimental design where researchers have no control over the variables and can only report on the variables as they naturally occur. Researchers look at the effect that independent variables, which have already occurred, have on the dependent variable.

4. Random Assignment: This is a process used in true experimental designs to ensure that each participant has an equal chance of being assigned to any group, treatment or control. It is a tool used to eliminate bias in the sample and distribute confounding variables evenly across all groups.

5. Longitudinal Research: This type of research involves repeated observations of the same variables or subjects over a period of time, which may span from years to even decades. While it is not focused on manipulating variables, it is aimed at observing changes and developmental trends over time.

In the context of health research on behavior or lifestyle factors, such as sexual behaviors or smoking, it is indeed unethical and often impractical to manipulate these behaviors experimentally. Hence, researchers often resort to quasi-experimental or ex post facto designs. These can be particularly informative, but they come with certain limitations, such as the inability to definitively establish causal relationships due to potential confounding variables that were not controlled for.