All of those calculations assume a random sample which this was not: "To reduce self-selection bias, invitations to complete surveys were sent out to members o

All of those calculations assume a random sample which this was not: "To reduce self-selection bias, invitations to complete surveys were sent out to members of Survation’s panel and the surveys were then conducted by online interview. This approach is far more rigorous than say, running a poll on a website in which anyone can choose to take part." Are "members of Survation's panel" a random subset of developers? How many invitations were sent out to get 238 responses (i.e. what was the response rate)? Are developers with burnout more likely to respond to a survey about burnout? Was the sample population demographically similar to the overall population of developers?