Context: I am currently doing my undergraduate research and I am stuck on my analysis part. The study is a quantitative experiment, I had 17 participants performing a maitlands mobilization Grade 3 for 3 times at a time. What I was looking for is if all individuals could exert the same force consistently across the 3 times (i.e. checking for reliability).
Problem: I want to test the Inter-rater reliability and Intra-rater reliability of these participants.
For intra-rater reliability I decided to use ICC in SPSS - One way random and Inter-rater reliability a Two-way random looking at the single measures for both.
However for Intra-rater reliability some of my values came out negative.
Question: Should I be doing a different method? For example, ICC 3,1
How can I interpret the negative values if they are still useful?