Power analysis for detecting trends in linear regression is implemented following procedures in Gerrodette (1987; 1991).

```
powertrend(trend = 1, A1 = NULL, PSE = NULL, pserel = 1,
maxyrs = 3, pR = 100, step = 5, alpha = 0.05, tail = 2, graph = TRUE)
```

trend

1 = Linear, 2 = Exponential. Default = 1.

A1

the start year abundance. In actuality, it can be population size, productivity, diversity, mortality rate, etc.

PSE

the proportional standard error (SE(A)/A) = CV in Gerrodette (1987;1991).

pserel

the relationship between abundance and PSE: 1 = 1/sqrt(A1), 2 = constant, 3 = sqrt(A1). Default = 1.

maxyrs

the maximum number of samples or years to project start year abundance. Default = 3.

pR

the highest positive percent change to investigate. Default = 100.

step

the increment of the range of percent change to investigate. Default = 5.

alpha

the alpha level (Type I error) to use. Default = 0.05.

tail

type of tailed test: 1 = one-tailed, 2= two-tailed. Default = 2.

graph

logical specifying whether a graph of power versus percent change should be produced. Default is TRUE.

Dataframe containing columns of number of samples (`years`

), trend selected (`trend`

), the PSE (`pse`

),
alpha level (`alpha`

), tail of test (`tail`

), percent change (`R`

) over `maxyrs`

, and power (`power`

).

The probability that an upward or downward trend in abundance (power) will be detected is calculated using linear regression
given number of samples (`maxyrs`

), estimates of sample variability (`PSE`

) and abundance-PSE relationship (`pserel`

),
and percent rate of change. The program calculates power for each `step`

increment beginning at -100 percent for declining changes
and ending at `pR`

percent for increasing changes. See Gerrodette (1987;1991)
for full details. It is assumed that time intervals between samplings is equal.

Gerrodette, T. 1987. A power analysis for detecting trends. Ecology. 68(5): 1364-1372.

Gerrodette, T. 1991. Models for power of detecting trends - a reply to Link and Hatfield. Ecology 72(5): 1889-1892.

```
# NOT RUN {
powertrend(A1=1000,PSE=0.1)
# }
```

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