tailieunhanh - Statistical Methods in Medical Research - part 6

Các thiết lập dữ liệu, mà là của n kích thước, được sắp xếp theo thứ tự tăng dần của tuổi tác và các điểm 1 kn được lựa chọn (trong đó k là một phần nhỏ, thường là giữa 0A 05 và 0A 2). Đối với những điểm này, hồi quy tuyến tính đơn giản của biến vào độ tuổi được xác định và các chất thải được tính toán. | Reference ranges 405 rather than a single measure of central tendency. The data set which is of size n is arranged in ascending order of age and points 1 to kn are selected where k is a fraction usually between 0-05 and 0-2 . For these points the simple linear regression of the variable on age is determined and the residuals are calculated. The m selected percentiles of these residuals are determined by sorting and counting and the m percentiles for the variable found by adding the m residual percentiles to the fitted value of the regression calculated at the median of the kn ages. These percentiles are plotted against the median age and then the whole process is repeated using points 2 to kn 1. The process continues until the whole range of ages is covered. This approach means that no percentiles are computed for the range of ages covered by the smallest kkn points and the largest kkn points. Example Ultrasound measurements of the kidneys of 560 newborn infants were obtained in a study reported by Scott et al. 1990 . Measurements of the depth length and area of each kidney were made and interest was focused on the range of kidney sizes as certain pathologies are associated with larger kidneys. However the assessment of what is a large kidney depends on the size of the baby and to allow for this birth-weight- and head-circumferencespecific percentiles were derived using the HRY method. For each of depth and length the percentiles were estimated for the maximum of the measurements on the right and left kidney. Figure shows the data on maximum kidney depth plotted against birth weight. The seven percentiles namely 3rd 10th 25th 50th 75th 90th and 97th are estimated. The unsmoothed percentiles are computed using k 0-2 the results for the 3rd 50th and 97th percentiles are illustrated. The unsmoothed percentiles are smoothed using and with p 1 qữ 2 and Í I 1 the fitted percentiles are given by 0 1 594 0-220Z O-OlSz and 1 0-201 0-002zj. These .