# pwm.ub: Unbiased Sample Probability-Weighted Moments In lmomco: L-Moments, Censored L-Moments, Trimmed L-Moments, L-Comoments, and Many Distributions

## Description

Unbiased sample probability-weighted moments (PWMs) are computed from a sample. The β_r's are computed using

β_r = n^{-1} {n-1 \choose r}^{-1} ∑^n_{j=1} {j-1 \choose r} x_{j:n}\mbox{.}

## Usage

 `1` ```pwm.ub(x, nmom=5, sort=TRUE) ```

## Arguments

 `x` A vector of data values. `nmom` Number of PWMs to return (r = `nmom - 1`). `sort` Do the data need sorting? The computations require sorted data. This option is provided to optimize processing speed if presorted data already exists.

## Value

An R `list` is returned.

 `betas` The PWMs. Note that convention is the have a β_0, but this is placed in the first index `i=1` of the `betas` vector. `source` Source of the PWMs: “pwm.ub”.

## Note

Through a user inquiry, it came to the author's attention in May 2014 that some unrelated studies using PWMs in the earth-system sciences have published erroneous sample PWMs formula. Because lmomco is intended to be an authoritative source, here are some computations to further prove correctness with provenance:

 ```1 2 3 4 5 6 7 8 9``` ```"pwm.handbookhydrology" <- function(x, nmom=5) { x <- sort(x, decreasing = TRUE); n <- length(x); betas <- rep(NA, nmom) for(r in 0:(nmom-1)) { tmp <- sum(sapply(1:(n-r), function(j) { choose(n - j, r) * x[j] / choose(n - 1, r) })) betas[(r+1)] <- tmp/n } return(betas) } ```

and a demonstration with alternative algebra in Stedinger and others (1993)

 ```1 2 3 4 5 6 7 8``` ```set.seed(1) glo <- vec2par(c(123,1123,-.5), type="glo"); X <- rlmomco(100, glo) lmom2pwm(lmoms(X, nmom=5))\$betas # unbiased L-moments flipped to PWMs [1] 998.7932 1134.0658 1046.4906 955.8872 879.3349 pwm.ub(X, nmom=5)\$betas # Hosking and Wallis (1997) and Asquith (2011) [1] 998.7932 1134.0658 1046.4906 955.8872 879.3349 pwm.handbookhydrology(X) # ** alert reverse sort, opposite usually seen** [1] 998.7932 1134.0658 1046.4906 955.8872 879.3349 ```

W.H. Asquith

## References

Asquith, W.H., 2011, Distributional analysis with L-moment statistics using the R environment for statistical computing: Createspace Independent Publishing Platform, ISBN 978–146350841–8.

Greenwood, J.A., Landwehr, J.M., Matalas, N.C., and Wallis, J.R., 1979, Probability weighted moments—Definition and relation to parameters of several distributions expressable in inverse form: Water Resources Research, v. 15, pp. 1,049–1,054.

Stedinger, J.R., Vogel, R.M., Foufoula-Georgiou, E., 1993, Frequency analysis of extreme events: in Handbook of Hydrology, ed. Maidment, D.R., McGraw-Hill, Section 18.6 Partial duration series, mixtures, and censored data, pp. 18.37–18.39.

Hosking, J.R.M., 1990, L-moments—Analysis and estimation of distributions using linear combinations of order statistics: Journal of the Royal Statistical Society, Series B, v. 52, pp. 105–124.

`pwm.pp`, `pwm.gev`, `pwm2lmom`

## Examples

 `1` ```pwm <- pwm.ub(rnorm(20)) ```

### Example output

```
```

lmomco documentation built on March 14, 2020, 5:06 p.m.