1, the sample size is too small. If the sample size is too small, the separation of variables may lead to the reduction of samples, thus affecting the reliability of the results.

2. Research variables are inseparable. Some variables themselves are inseparable, such as gender, age, race, etc.

3. The research topic is not suitable for separating variables. Some research topics need to put multiple variables together to get more comprehensive results.

4. The study design is not suitable for separating variables. Some research designs may need to consider multiple variables at the same time, such as longitudinal studies or cross-sectional studies.

5. There is interaction between the research variables. If there is interaction between variables, separating variables may lead to ignoring this interaction, thus affecting the accuracy of the results.