Tuesday, June 17, 2014

Setting up a cruise: Bringing the pieces together

In recent posts we’ve talked about how the set-up of a cruise is influenced by your requirements for confidence and error, and the variation you anticipate in the stand.  The total number of plots in a cruise is calculated using a standard sample size formula, which you might remember from a forest measurements class:

n = ( t * CV )2

Or, in simpler terms:

number of plots = ( t-statistic * Coefficient of variation )2
Allowable error

The t-statistic comes from the level of confidence you specify.

The criteria you specify are used in that equation to determine the final number of plots- generally, here’s how that works:

More plots:
Fewer plots:
Higher confidence required
Lower confidence required

Because the t-statistic (reflecting confidence) and the estimated variation are multiplied, if you require a high level of confidence in results from cruising a highly variable stand, the number of plots required will be much larger than if you need an estimate with lower confidence, or if the stand is less variable. The final plots are then located in a grid across the stand. 

Once you know where to go, the next challenge becomes how to decide what data to collect and what plot design to use- fixed area plots, or strips of varying sizes, or a variable-radius plot.  We’ll get into that in a future series of posts!

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