johnson.blight {agridat} | R Documentation

## Potato blight due to weather in Prosser, Washington

### Description

Potato blight due to weather in Prosser, Washington

### Format

A data frame with 25 observations on the following 6 variables.

`year`

  year

`area`

  area affected, hectares

`blight`

  blight detected, 0/1 numeric

`rain.am`

  number of rainy days in April and May

`rain.ja`

  number of rainy days in July and August

`precip.m`

  precipitation in May when temp > 5C, milimeters

### Details

The variable 'blight detected' is 1 if 'area' > 0.

### Source

Johnson, D.A. and Alldredge, J.R. and Vakoch, D.L. (1996). Potato late blight forecasting models for the semiarid environment of south-central Washington. Phytopathology, 86, 480–484. http://doi.org/10.1094/Phyto-86-480

### References

Vinayanand Kandala, Logistic Regression

### Examples

    data(johnson.blight)
    dat <- johnson.blight

    # Define indicator for blight in previous year
    dat$blight.prev[2:25] <- dat$blight[1:24]
    dat$blight.prev[1] <- 0 # Need this to match the results of Johnson
    dat$blight.prev <- factor(dat$blight.prev)
    dat$blight <- factor(dat$blight)

    # Johnson et al developed two logistic models to predict outbreak of blight

    m1 <- glm(blight ~ blight.prev + rain.am + rain.ja, data=dat, family=binomial)
    summary(m1)
    ##              Estimate Std. Error z value Pr(>|z|)
    ## (Intercept)  -11.4699     5.5976  -2.049   0.0405 *
    ## blight.prev1   3.8796     1.8066   2.148   0.0318 *
    ## rain.am        0.7162     0.3665   1.954   0.0507 .
    ## rain.ja        0.2587     0.2468   1.048   0.2945
    ## ---
    ## Signif. codes:  0 '***' 0.001 '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1

    ## (Dispersion parameter for binomial family taken to be 1)

    ##     Null deviance: 34.617  on 24  degrees of freedom
    ## Residual deviance: 13.703  on 21  degrees of freedom
    ## AIC: 21.703

    m2 <- glm(blight ~ blight.prev + rain.am + precip.m, data=dat, family=binomial)
    summary(m2)
    ##              Estimate Std. Error z value Pr(>|z|)
    ## (Intercept)   -7.5483     3.8070  -1.983   0.0474 *
    ## blight.prev1   3.5526     1.6061   2.212   0.0270 *
    ## rain.am        0.6290     0.2763   2.276   0.0228 *
    ## precip.m      -0.0904     0.1144  -0.790   0.4295
    ## ---
    ## Signif. codes:  0 '***' 0.001 '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1

    ## (Dispersion parameter for binomial family taken to be 1)

    ##     Null deviance: 34.617  on 24  degrees of freedom
    ## Residual deviance: 14.078  on 21  degrees of freedom
    ## AIC: 22.078

    require(lattice)
    splom(dat[,c('blight','rain.am','rain.ja','precip.m')],
          main="johnson.blight - indicator of blight")

[Package agridat version 1.16 Index]