R: One-sample test of a (standardised) major axis elevation

elev.test {smatr} | R Documentation

## One-sample test of a (standardised) major axis elevation

### Description

Test if the elevation of a major axis or standardised major axis equals a specific value. This can now be done via `sma(y~x,elev.test=0)`, see help on the `sma` function.

### Usage

        elev.test(y, x, test.value = 0, data = NULL, alpha = 0.05,
            method = 'SMA', robust = FALSE, V = matrix(0,2,2) )

### Arguments

`y` | The Y-variable
`x` | The X-variable
`test.value` | The hypothesised value of the elevation (default value is 0)
`data` | Deprecated. Use with() instead (see Examples).
`alpha` | The desired confidence level for the 100(1-alpha)% confidence interval for the common slope. (Default value is 0.05, which returns a 95% confidence interval.)
`method` | The line fitting method: 'OLS' or 0 linear regression 'SMA' or 1 standardised major axis (this is the default) 'MA' or 2 major axis
`robust` | If TRUE, uses a robust method to fit the lines and construct the test statistic.
`V` | The estimated variance matrix of measurement error. Default is that there is no measurement error.

### Details

Tests if the line relating y to x has an elevation equal to test.value (which has a default value of 0). The line can be a linear regression line, major axis or standardised major axis (as selected using the input argument choice). The test is carried out usinga t-statistic, comparing the difference between estimated and hypothesised elevation to the standard error of elevation. As described in Warton et al (2006).

A confidence interval for the elevation is also returned, again using the t-distribution.

If measurement error is present, it can be corrected for through use of the input argument V, which makes adjustments to the estimated sample variances and covariances then proceeds with the same method of inference. Note, however, that this method is only approximate (see Warton et al 2006 for more details).

The test assumes the following:

1. y and x are linearly related

2. residuals independently follow a normal distribution with equal variance at all points along the line

The assumptions can be visually checked by plotting residual scores against fitted axis scores, and by constructing a Q-Q plot of residuals against a normal distribution, available using the `plot.sma` function.

Setting `robust=TRUE` fits lines using Huber's M estimation, and modifies the test statistic as proposed in Taskinen & Warton (in review).

### Value

`t` | The test statistic (a t-statistic).
`p` | The P-value, taken from the t_(n-2)-distribution. This is an exact test if residuals are normally distributed.
`test.value` | The hypothesised value of the elevation.
`a` | The estimated elevation.
`ci` | A 100(1-alpha)% CI for the slope.

### Author(s)

Warton, D.I.David.Warton@unsw.edu.au, J. Ormerod, & S. Taskinen

### References

Warton D. I., Wright I. J., Falster D. S. and Westoby M. (2006) A review of bivariate line-fitting methods for allometry. Biological Reviews 81, 259–291.

Taskinen, S. and D.I. Warton. in review. Robust tests for one or more allometric lines.

### See Also

`sma`, `line.cis`, `slope.test`

### Examples

    #load the leaflife dataset:
    data(leaflife)

    #consider only the low rainfall sites:
    leaf.low.rain <-  subset(leaflife, rain=="low")

    #construct a plot
    plot(log10(leaf.low.rain$lma), log10(leaf.low.rain$longev),
       xlab="leaf mass per area [log scale]", ylab="leaf longevity [log scale]")

    #test if the SMA elevation is 0 for leaf longevity vs LMA
    with(leaf.low.rain, elev.test(log10(lma), log10(longev)))

    #test if the MA elevation is 2
    with(leaf.low.rain,elev.test(log10(lma), log10(longev),
       test.value = 2, method = "MA"))

[Package smatr version 3.4-3 Index]