Comparison of survival curves between two groups is an important part of disease prognosis study.Log-rank test is commonly used,but when the two curves' later intersecting opening is too large,the proportion of assumptions is not established,thus the Log-rank test is ineffective.We introduces five statistical tests to compare two survival curves at a fixed time points:classic method,logarithmic transformation,cloglog transformation,arcsine transformation and logit transformation.Through the study we found that if the overall survival curves are difficult to compare between groups tested with Log-rank test method or Two-stage test method,the fixed time point test can effectively determine whether there was significant difference in survival rate at a fixed time point.Among the five fixed time point tests,cloglog transformation could give more precise result.

Objective : To investigate the protective effect of melatonin (MT) on formaldehyde (FA) inhalation-in- duced acute lung injury (ALI) in rats and its mechanism through the regulation of nuclear factor E2-related factor 2 (Nrf2) signaling pathway． Methods : Fifty female Wistar rats were randomly divided into Control group ，FA group，FA + MT 5 mg / kg group，FA + MT 10 mg / kg group and FA + MT 20 mg / kg group，with 10 rats in each group．Except for the Control group，all other groups inhaled 3 mg / m3 FA daily for 21 d consecutively to construct the tainted model，and then treated with different MT doses for 14 d．The tainting was continued during the MT treatment．Hematoxylin-eosin (HE) staining was used to observe the histopathological changes in lung tissue，lung water content and lung coefficient were weighed and measured，glutathione ( GSH) ，superoxide dismutase (SOD) and 8-hydroxydeoxyguanosine ( 8-OHdG) levels were measured by absorbance photometric method ，and enzyme linked immunosorbent assay(ELISA) was used to measure the levels of tumor necrosis factor-alpha (TNF-α) ，in- terleukin (IL) -6，and IL-1 β concentrations，Western blot to detect the protein expression levels of Nrf2，heme ox- ygenase-1 (HO-1) ，nuclear factor-κB ( NF-κB) ，and phosphorylated nuclear factor-κB ( p-NF-κB) in lung tis- sues，and quantitative polymerase chain reaction(qPCR) to detect the Nrf2，HO-1，and Kelch-like ECH-associated protein 1 (Keap1) mRNA expression levels. Results : Compared with the control group，lung injury was obvious in rats in the FA group ; lung tissue GSH and SOD levels were reduced ，and 8-OHdG levels were elevated ( P < 0. 05) ; alveolar lavage fluid TNF-α , IL-6，and IL-1 β levels were elevated (P＜0. 05) ; Nrf 2 and HO-1 protein expression levels were reduced in the lung tissue (P＜0. 05) ，and p-NF-κB protein expression levels were was ele- vated (P＜0. 05) ; the relative mRNA expression of Nrf2 and HO-1 in lung tissue was decreased，and the relative mRNA expression of Keap1 was elevated (P＜0. 05) ．Compared with the FA group，the lung injury of rats in the MT group was improved ; the levels of GSH and SOD in the lung tissue were increased (P＜0. 05) ，and the level of 8-OHdG was decreased (P＜0. 05) ; the levels of TNF-α , IL-6，and IL-1 β in the alveolar lavage fluid were de- creased (P＜0. 05) ; and the expression levels of the Nrf2 and HO-1 proteins in the lung tissue were increased (P ＜0. 05) ．p-NF-κB protein expression level was decreased (P ＜0. 05) ; the relative mRNA expression levels of Nrf2 and HO-1 in lung tissues were increased (P＜0. 05) ，and the relative mRNA expression level of Keap1 was decreased (P＜0. 05) in lung tissues，and all of them were in a dose-dependent manner． Conclusion MT can al- leviate oxidative stress and inflammatory responses and mitigate FA exposure-induced acute lung injury by regula- ting the Nrf2 / Keap1 / HO-1 signaling pathway.