Each sampling method suffers from a form of bias. Identify the bias, tell how the sample statistic might differ significantly from the population parameter, and (writing to learn) suggest how the problem might be corrected.
A group seeking taxpayer support for a new playground sends pollsters to all the local PTA meetings to ask, "Do you support the use of city taxes to fund a new park that will provide a safe and convenient recreation facility for your children?" A gratifying 88$\%$ respond yes.