In Katia's remedial mathematics study, she collected pretest data from a group of participants. She divided the participants into three groups. One group received no treatment, one group received instruction by a teacher-delivered intervention, one group received peer tutoring on the same problems. After the intervention she tested their math skills. With which validity threat should Katia be most concerned?
◦ Pretest-treatment interaction
◦ Instrumentation
◦ Testing
◦ Maturation