Important sampling question

Hi Folks



I was asked an important question about sampling. Does anyone know what sample size and make up a good sample needs to have when testing for usability for people with cognitive and learning disabilities?



What do people normally do?



Are there any research papers on this? What has been found to be reliable and





All the best
Lisa Seeman

http://il.linkedin.com/in/lisaseeman/, https://twitter.com/SeemanLisa