在统计学中，样本方差的计算公式是？

快手平台如何支持用户获得收入？

在心理咨询过程中，以下哪种做法最有助于案主对自己的婚姻责任进行反思？

对于二阶线性非齐次微分方程，若已知齐次方程的两个线性无关解y1和y2，如何构造通解？

A researcher conducts a survey to understand public opinion on a newly implemented policy. The survey results indicate that 70% of respondents support the policy. However, the researcher later discovers that the survey was primarily distributed through an online platform that is mainly used by a specific demographic group that is known to be more likely to support the policy. What is the most significant limitation of this survey's findings?