Answer:a small sample size
Explanation:
From the information given in the question, we are informed that Sandra developed a program that was designed to increase reading comprehension scores in 6th graders and she then randomly selects three students and has them participate in her program.
After comparing the mean score from the students who completed her program to three students who did not participate in the program, she finds no significant difference between their mean scores, it should be noted that the reason for this will be because the sample size is small.
If a large sample size was used, this could have been different as thsi could result in a significant difference between their mean scores.