Graham recruited student volunteers to participate in his dissertation study. he set up several times for students to come to a specified classroom and read various types of instructional materials and to be tested. he ran all of his control conditions first and then for each session he placed all students at the session in the same treatment condition. this is problematic because those who volunteered early are likely different than those who volunteered later. this problem is primarily due to

1. response bias.
2. instrumentation bias.
3. researcher bias.
4. sampling bias.