"Having the full sample of charter schools was essential for that type of work," Dr. Ladd said.
In other words, the study looked at a limited sample of charter school students, excluding those most likely to be academically challenged, and still found that only a minority of those students outperformed their traditional public school counterparts, and only for some of the years studied.
Second, a total of 457 patient days (20 included patients) is a small sample, although Charter and Feldt [ 44] argue that 'it is not theoretically defensible to set a universal standard for test score reliability'.
In the future, however, the National Center for Education Statistics, which conducts the survey, will cover only a random sample of about 300 charter schools.
Northampton County public schools out-performed a sampling of cyber and charter schools in every category of the Pennsylvania System of School Assessment exams in 2010.
Consequently a convenience sample of privately owned chartered physiotherapy practices was selected, which represented diverse locations throughout the Republic of Ireland, including both urban and rural settings.
The national center also switched its methodology to include visiting each charter school in the sample.
The research only looked at 1,372 schools, a small sample compared with the number of charter schools nationally.
But in an e-mail message to Dr. Fuller, Kerry Gruber, the author, wrote that for the next survey, researchers had canvassed a random sample of about 10percentt of about 3,000 charter schools.
As was suggested by Charter [ 32], the minimum sample size needs to be larger than 400 to evaluate reliability and validity.
