Abstract: Objective To analyze the influence of different sample size on phenomenon interpretation of qualitative research, improve the research ideas and the quality of health research report. Methods The subjects were divided into purpose sampling group and open sampling group and the number of subjects in the two groups was different. The interviews were conducted in full accordance with the qualitative research norms. The research was supervised by clinical experts and the final reports were evaluated and discussed. Results The same topic was extracted in the two groups, but the experience of the objective sampling group are easy to lead to misunderstanding, confusion or lack. There was close relationship between the experience of the interviewees and its meaning in open sampling group and the results were more serious and prominent in clinical significance, while in purpose sampling group, it is easy to misunderstand or confuse the experience. Conclusion Comparatively speaking, purposive sampling is not conducive to the full explanation of the phenomenon and exerts negative influence on the search for complexity and the proposal of new problems, resulting in the popularization of the results. The purposive sampling is recommended for qualitative research with high homogeneity, and open sampling is for qualitative research with high heterogeneity.

Key words: qualitative research, mechanical ventilation, purpose sampling, open sampling