Sampling Frame
Clearly describe how you will choose the sample. In quantitative studies, random sampling is nearly ALWAYS better than trying to sample the entire population (which allows for self selection and therefore adding bias to your study). See the Sample Size Calculator to determine the appropriate sample size for your population.
Explain the sampling method (e.g., simple random sampling, stratified random sampling, convenience sampling) and clearly explain the parameters used.
Discuss who has been excluded from the sample and why. If your selected subjects/participants opt out of the study, that is not the same as delimiting the study (selecting the type of person you will include). Self-selecting out of the study adds bias and you must carefully select replacement subjects/participants to replace those who self-select out of the study.
Note that your original sample consists of all potential participants you approach to participate; the final sample consists of those who actually DO PARTICIPATE.