---
title: "Life Data Analysis Part I - Estimation of Failure Probabilities"
subtitle: "A Non-parametric Approach"
author:
- "Tim-Gunnar Hensel"
- "David Barkemeyer"
date: "`r Sys.Date()`"
output:
rmarkdown::html_vignette:
fig_height: 6
fig_width: 7
fig_caption: yes
vignette: >
%\VignetteEngine{knitr::rmarkdown}
%\VignetteIndexEntry{Life Data Analysis Part I - Estimation of Failure Probabilities}
%\VignetteEncoding{UTF-8}
---
```{r setup, echo=FALSE, message=FALSE}
knitr::opts_chunk$set(
collapse = TRUE,
screenshot.force = FALSE,
comment = "#>"
)
library(weibulltools)
```
This document presents non-parametric estimation methods for the computation of failure probabilities of complete data (failures) taking (multiple) right-censored units into account. A unit can be either a single component, an assembly or an entire system.
Furthermore, the estimation results are presented in distribution-specific probability plots.
## Introduction to Life Data Analysis
If the lifetime (or any other damage-equivalent quantity such as distance or load cycles) of a unit is considered to be a continuous random variable _T_, then the probability that a unit has failed at _t_ is defined by its _CDF (cumulative distribution function)_ _F(t)_.
$$ P(T\leq t) = F(t) $$
In order to obtain an estimate of the _CDF_ for each observation $t_1, t_2, ..., t_n$ two approaches are possible. Using a parametric lifetime distribution requires that the underlying assumptions for the sample data are valid. If the distribution-specific assumptions are correct, the model parameters can be estimated and the _CDF_ is computable. But if assumptions are not held, interpretations and derived conclusions are not reliable.
A more general approach for the calculation of cumulative failure probabilities is to use non-parametric statistical estimators $\hat{F}(t_1), \hat{F}(t_2), ..., \hat{F}(t_n)$. In comparison to a parametric distribution no general assumptions must be held. For non-parametric estimators, an ordered sample of size $n$ is needed. Starting at $1$, the ranks $i \in \{1, 2, ..., n \}$ are assigned to the ascending sorted sample values. Since there is a known relationship between ranks and corresponding ranking probabilities a _CDF_ can be determined.
But rank distributions are systematically skewed distributions and thus the median value instead of the expected value $E\left[F\left(t_i\right)\right] = \frac{i}{n + 1}$ is used for the estimation [^note1]. This skewness is visualized in _Figure 1_.
[^note1]: Kapur, K. C.; Lamberson, L. R.: _Reliability in Engineering Design_,
_New York: Wiley_, 1977, pp. 297-301
```{r rank_densities, fig.cap = "Figure 1: Densities for different ranks i in samples of size n = 10.", message = FALSE, warning = FALSE}
library(dplyr) # data manipulation
library(ggplot2) # visualization
x <- seq(0, 1, length.out = 100) # CDF
n <- 10 # sample size
i <- c(1, 3, 5, 7, 9) # ranks
r <- n - i + 1 # inverse ranking
df_dens <- expand.grid(cdf = x, i = i) %>%
mutate(n = n, r = n - i + 1, pdf = dbeta(x = x, shape1 = i, shape2 = r))
densplot <- ggplot(data = df_dens, aes(x = cdf, y = pdf, colour = as.factor(i))) +
geom_line() +
scale_colour_discrete(guide = guide_legend(title = "i")) +
theme_bw() +
labs(x = "Failure Probability", y = "Density")
densplot
```
### Failure Probability Estimation
In practice, a simplification for the calculation of the median value, also called median rank, is made. The formula of _Benard's_ approximation is given by
$$\hat{F}(t_i) \approx \frac{i - 0,3}{n + 0,4} $$
and is described in _The Plotting of Observations on Probability Paper_ [^note2].
[^note2]: Benard, A.; Bos-Levenbach, E. C.: _The Plotting of Observations on Probability Paper_,
_Statistica Neerlandica 7 (3)_, 1953, pp. 163-173
However, this equation only provides valid estimates for failure probabilities if all units in the sample are defectives (`estimate_cdf(methods = "mr", ...)`).
In field data analysis, however, the sample mainly consists of intact units and only a small fraction of units failed. Units that have no damage at the point of analysis and also have not reached the operating time or mileage of units that have already failed, are potential candidates for future failures. As these, for example, still are likely to fail during a specific time span, like the guarantee period, the _CDF_ must be adjusted upwards by these potential candidates.
A commonly used method for correcting probabilities of (multiple) right-censored data is _Johnson's_ method (`estimate_cdf(methods = "johnson", ...)`). By this method, all units that are included in the period looked at are sorted in an ascending order of their operating time (or any other damage-equivalent quantity). If there are units that have not failed before the _i_-th failure, an adjusted rank for the _i_-th failure is formed. This correction takes the potential candidates into account and increases the rank number. In consequence, a higher rank leads to a higher failure probability. This can be seen in _Figure 1_.
The rank adjustment is determined with:
$$j_i = j_{i-1} + x_i \cdot I_i, \;\; with \;\; j_0 = 0$$
Here, $j_ {i-1}$ is the adjusted rank of the previous failure, $x_i$ is the number of defectives at $t_i$ and $I_i$ is the increment that corrects the considered rank by the potential candidates.
$$I_i=\frac{(n+1)-j_{i-1}}{1+(n-n_i)}$$
The sample size is $n$ and $n_i$ is the number of units that have a lower $t$ than the _i_-th unit. Once the adjusted ranks are calculated, the _CDF_ can be estimated according to _Benard's_ approximation.
Other methods in {weibulltools} that can also handle (multiple) right-censored data are the _Kaplan-Meier_ estimator (`estimate_cdf(methods = "kaplan", ...)`) and the _Nelson-Aalen_ estimator (`estimate_cdf(methods = "nelson", ...)`).
### Probability Plotting
After computing failure probabilities a method called _probability plotting_ is applicable. It is a graphical _goodness of fit_ technique that is used in assessing whether an assumed distribution is appropriate to model the sample data.
The axes of a probability plot are transformed in such a way that the _CDF_ of a specified model is represented through a straight line. If the plotted points (`plot_prob()`) lie on an approximately straight line it can be said that the chosen distribution is adequate.
The two-parameter Weibull distribution can be parameterized with parameters $\mu$ and $\sigma$ such that the _CDF_ is characterized by the following equation:
$$F(t)=\Phi_{SEV}\left(\frac{\log(t) - \mu}{\sigma}\right)$$
The advantage of this representation is that the Weibull is part of the (log-)location-scale family. A linearized representation of this _CDF_ is:
$$\Phi^{-1}_{SEV}\left[F(t)\right]=\frac{1}{\sigma} \cdot \log(t) - \frac{\mu}{\sigma}$$
This leads to the following transformations regarding the axes:
* Abscissa: $x = \log(t)$
* Ordinate: $y = \Phi^{-1}_{SEV}\left[F(t)\right]$, which is the quantile function
of the SEV (_smallest extreme value_) distribution and can be written out with
$\log\left\{-\log\left[1-F(t)\right]\right\}$.
Another version of the Weibull _CDF_ with parameters $\eta$ and $\beta$ results in a _CDF_ that is defined by the following equation:
$$F(t)=1-\exp\left[ -\left(\frac{t}{\eta}\right)^{\beta}\right]$$
Then a linearized version of the CDF is:
$$ \log\left\{-\log\left[1-F(t)\right]\right\} = \beta \cdot \log(t) - \beta \cdot \log(\eta)$$
Transformations regarding the axes are:
* Abscissa: $x = \log(t)$
* Ordinate: $y = \log\left\{-\log\left[1-F(t)\right]\right\}$.
It can be easily seen that the parameters can be converted into each other. The corresponding equations are:
$$\beta = \frac{1}{\sigma}$$
and
$$\eta = \exp\left(\mu\right).$$
## Data: Shock Absorber
To apply the introduced methods of non-parametric failure probability estimation and probability plotting the `shock` data is used. In this dataset kilometer-dependent problems that have occurred on shock absorbers are reported. In addition to failed items the dataset also contains non-defectives (*censored*) observations. The data can be found in _Statistical Methods for Reliability Data_ [^note3].
[^note3]: Meeker, W. Q.; Escobar, L. A.: _Statistical Methods for Reliability Data_,
_New York, Wiley series in probability and statistics_, 1998, p. 630
For consistent handling of the data, {weibulltools} introduces the function `reliability_data()` that converts the original dataset into a `wt_reliability_data` object. This formatted object allows to easily apply the presented methods.
```{r dataset_shock, message = FALSE}
shock_tbl <- reliability_data(data = shock, x = distance, status = status)
shock_tbl
```
## Estimation of Failure Probabilities with {weibulltools}
First, we are interested in how censored observations influence the estimation of failure probabilities in comparison to the case where only failed units are considered. To deal with survived and failed units we will use `estimate_cdf()` with `methods = "johnson"`, whereas `methods = "mr"` only considers failures.
```{r failure_probabilities}
# Estimate CDF with both methods:
cdf_tbl <- estimate_cdf(shock_tbl, methods = c("mr", "johnson"))
# First case where only failed units are taken into account:
cdf_tbl_mr <- cdf_tbl %>% filter(cdf_estimation_method == "mr")
cdf_tbl_mr
# Second case where both, survived and failed units are considered:
cdf_tbl_john <- cdf_tbl %>% filter(cdf_estimation_method == "johnson")
cdf_tbl_john
```
If we compare both outputs we can see that survivors reduce the probabilities. But this is just that what was expected since undamaged units with longer or equal lifetime characteristic _x_ let us gain confidence in the product.
## Probability Plotting with {weibulltools}
The estimated probabilities should now be presented in a probability plot. With `plot_prob()` probability plots for several lifetime distributions can be constructed and estimates of multiple methods can be displayed at once.
### Weibull Probability Plot
```{r probability_plot_weibull, fig.cap = "Figure 3: Plotting positions in Weibull grid.", message = FALSE}
# Weibull grid for estimated probabilities:
weibull_grid <- plot_prob(
cdf_tbl,
distribution = "weibull",
title_main = "Weibull Probability Plot",
title_x = "Mileage in km",
title_y = "Probability of Failure in %",
title_trace = "Method",
plot_method = "ggplot2"
)
weibull_grid
```
_Figure 3_ shows that the consideration of survivors (orange points, _Method: johnson_) decreases the failure probability in comparison to the sole evaluation of failed items (green points, _Method: mr_).
### Log-normal Probability Plot
Finally, we want to use a log-normal probability plot to visualize the estimated failure probabilities.
```{r probability_plot_log-normal, fig.cap = "Figure 4: Plotting positions in log-normal grid.", message = FALSE}
# Log-normal grid for estimated probabilities:
lognorm_grid <- plot_prob(
cdf_tbl,
distribution = "lognormal",
title_main = "Log-normal Probability Plot",
title_x = "Mileage in km",
title_y = "Probability of Failure in %",
title_trace = "Method",
plot_method = "ggplot2"
)
lognorm_grid
```
On the basis of _Figure 3_ and _Figure 4_ we can subjectively assess the goodness of fit of Weibull and log-normal. It can be seen that in both grids, the plotted points roughly fall on a straight line. Hence one can say that the Weibull as well as the log-normal are good model candidates for the `shock` data.