I have set up an experiment that tests 3 instruments on 35 different people. Each person gets tested three times with each instrument (see below). Now i understand this is paired data, i was just wondering whether there is a statistical test to check for equality of variance. From Googling i understand Levene's test is generally used for independent samples but i can't find any test that can do the same for comparing paired data.

At the moment i have used the pooled std Dev for each instrument to say that with 95% confidence (1.97 SD) that any measure i take will be within say 3cm with instrument 1 , 2cm with instrument 2 and 1cm with instrument 3. But i would like to statistically test whether my data has equality of variance.

I hope to be able to say something along the lines of ...... their is decreased variability between measures with instrument 3 than 1 and 3. Therefore choose instrument 3.

Now i have plotted some Bland Altman plots of the first and third repeated measures for each instrument and these show the variability. Is that enough to show variability or do you think i should do some sort of significance test?

\(

\begin{tabular}{cccc|ccc|ccc}

Instrument & & 1 & \multicolumn{1}{c}{} & & 2 & \multicolumn{1}{c}{} & & 3 & \tabularnewline

Subject-Reps & 1 & 2 & \multicolumn{1}{c}{3} & 1 & 2 & \multicolumn{1}{c}{3} & 1 & 2 & 3\tabularnewline

\hline

Tom & 4 & 4 & 5 & 7 & 8 & 6 & 4 & 5 & 5\tabularnewline

Alec & 13 & 14 & 16 & 11 & 11 & 11 & 12 & 17 & 10\tabularnewline

\end{tabular}

\)