We must select the type of observational or interventional design ( cross-sectional, case-control, cohort, experimental, or ecological design, depending on our research question, the type of exposure and outcome variables.

It should be noted our target population and the population sample and sampling procedure which we will use.

It should be noted the sample size calculation and that we use to calculate it.

It should be noted that the statistical analysis we anticipate that we will use to analyze the data that are collected.

Ethical and legal considerations, we should note as maintain confidentiality, how to obtain informed consent and what Bioethics Committee will review the protocol.

In your protocol, should be the calculations of sample size with precision and power.

Always include a biostatistics expert since the begining of the writing of protocol. You should be include statistical analysis complete, before to collect any data.

Also, you should include a control of quality of data, training researcher associated, etc.