Use nonparametric growth curves to analyze data from a repairable system without making assumptions about the distribution of successive failure (or repair) times.
You can estimate growth curves of the mean cost of maintaining the system or the mean number of repairs over time. You can then determine whether a trend exists in times or costs between successive failures and whether system failures or costs are becoming more frequent, less frequent, or remaining constant.
A reliability engineer wants to compare the failure rate for two different types of a brake component that is used on subway trains. The engineer collects replacement time data and component type for 29 trains. Each time a unit failed, it was repaired and returned to service.