PEFR MEASURED WITH WRIGH PEAK FLOW AND MINI WRIG:T PEAK LOW METER Wright peak flow meter Mini Wright peak flow meter First pefr Second pefr first pefr Second pefr ubject (1/min) (1/min) (1/min) 525 395 397 430 415 516 512 508 476 500 6 611 600 625 7 413 415 364 431 650 638 658 642 433 445 432 11 417 420 432 420 12 656 633 626 275 478 477 178 165 259 268 372 370 17 421 451 443 已00 5 400 200 0 50o R by large meter (l/min) Fig 1. PEFR measured with large Wright peak flow meter and mini Wright peak flow meter with line of equality INAPPROPRIATE USE OF CORRELATION COEFFICIENT The second step is usually to calculate the correlation coefficient(r)between the two methods. For the data in fig 1, r=0.94(p<0.001). The null hypothesis here is that the measurements by the two methods are not linearly related. The probability is very small and we can safely conclude that PEFR measurements by the mini and large meters are related However this high correlation does not mean that the two methods agree (1)r measures the strength of a relation between two variables, not the agreement between them. We have perfect agreement only if the points in fig 1 lie along the line of equality, but we will have perfect correlation if the points lie along any straight line2 PEFR MEASURED WITH WRIGHT PEAK FLOW AND MINI WRIGHT PEAK FLOW METER Wright peak flow meter Mini Wright peak flow meter First PEFR Second PEFR First PEFR Second PEFR Subject (l/min) (l/mi) (l/min) (l/min) 1 494 490 512 525 2 395 397 430 415 3 516 512 520 508 4 434 401 428 444 5 476 470 500 500 6 557 611 600 625 7 413 415 364 460 8 442 431 380 390 9 650 638 658 642 10 433 429 445 432 11 417 420 432 420 12 656 633 626 605 13 267 275 260 227 14 478 492 477 467 15 178 165 259 268 16 423 372 350 370 17 427 421 451 443 Fig 1. PEFR measured with large Wright peak flow meter and mini Wright peak flow meter, with line of equality. INAPPROPRIATE USE OF CORRELATION COEFFICIENT The second step is usually to calculate the correlation coefficient (r) between the two methods. For the data in fig 1, r = 0.94 (p < 0.001). The null hypothesis here is that the measurements by the two methods are not linearly related. The probability is very small and we can safely conclude that PEFR measurements by the mini and large meters are related. However, this high correlation does not mean that the two methods agree: (1) r measures the strength of a relation between two variables, not the agreement between them. We have perfect agreement only if the points in fig 1 lie along the line of equality, but we will have perfect correlation if the points lie along any straight line