Thanks Miner for your reply and i have read your excellent blog . my question is why cant i use 2 sample t-test instead of youden plot obland altman plot .
Because that test isn't intended for the type of data you have.
first you have matched pairs not grouped pairs.
second your data is NOT independent as it is repeated measures of the same things - which is a requirement for a 2 sample t test or standard correlation statistics for that matter.
Beyond the incorrectness of the statistical tool the answer can be wrong as a systemic bias between instruments will NOT be seen in a grouped analysis unless it is ridiculously large but will clearly be seen in a matched pair analysis (Youden and Bland-Altman)
See the attached spreadsheet for an example. There is a bias AND it passes a two sample t test. Note too that the individual instrument gage R&Rs would "pass".
The bias isn't huge, but the bland altman does properly quantify it so a rational decision can be made regarding it's impact.