This morning, I reviewed a survey entitled, Americans Continue to Adjust Their Ideal Weight Upward, from the Wellbeing section of Gallup. The survey found that Americans' self-reported actual weight has steadily increased since 1990.
The study shows that the average weight for Americans is 176 pounds and the average ideal weight is 162 pounds.

The study was conducted via telephone and included questions regarding dieting habits, and opinions about constituents' current and ideal weights. The telephone interviews were conducted Nov. 15-18 of last year and randomly sampled 1,015 adults using random-digit sampling.
Although I am not concerned with the sampling error in this survey, I would be curious about how many people report their actual weight correctly and whether telephone sampling is the best method for surveys related to weight or sensitive health topics.
In an effort to understand, I consulted the Journal of Ethnicity & Disease, which dissected data from the 2007-2008 National Health and Nutrition Examination Survey which surveyed 2,672 men and 2,671 women on height, weight, and BMI.
The researcher, Doctor Lori Kowaleski-Jones, found that both men and women overestimate height and women tend to underestimate their weight.
With this being said if I were to conduct a survey that is dependent on exact weights and heights and time and money weren't factors, I would conduct this survey in-person and have a medical professional conduct physical assessments of weight and height before asking questions about ideal weight and opinions. Thus, eliminating inaccurate data.