Many of us have been faced with the prospect of recommending or conducting a check cruise. There are a number of reasons to do a check cruise, but three sort to the top of the list: 1) to verify field procedures and measurement quality; 2) to assess compliance with contract/cruise specifications; and 3) to determine if an adjustment of an existing cruise is needed and to make the adjustment if needed. To conduct a successful check cruise, the demands of each objective need to be examined and care taken to collect the correct information. I will address each of these objectives separately, pointing out important design features. Remember, in most cases, your check cruise should be considered a sample - all plots/trees must have a chance of being checked.
Field Procedure and Measurement Quality Assessment
To assess the quality of the measurements being taken, some obvious prerequisites come to mind:
Assessing measurement quality serves two valuable functions: verifying adherence to policy and training. Check cruise results should be shared with the crew soon after the original cruise. Errors that where found can be discussed and any trends in them can be flushed out. You may find that your procedures need refinement or clarification, or that more training is required.
Compliance with Contract/Cruise Specifications
In most cases, your check cruise will be a subsample of the original cruise. Therefore, you will only have an estimate of the quality of the cruise. Additionally, you may find many kinds of errors (miscalled in-trees, height measurement errors, species miscalls). To make sense of all this, you should think about the objective - a reliable estimate of the volume and products of the tract perhaps - and set your check specifications accordingly. One method is to conduct a statistically valid sample of original measurement plots. Compute the check cruise volume of each plot. You can then employ a Paired t-Test to compare the check volumes against the original. Using a t-Test makes it clear that you have a sample; that the confidence of your conclusions depends on sample size among other things. (The t-Test or other comparison tool emphasizes the variability in each sample (the original and check) and gives you a method for estimating how many check plots are needed to detect differences.) Using volume as the attribute for comparison integrates the effect of any errors found and weights them appropriately. Other characteristics, such as value, could be used also.
But let's back up a minute. Are there some ways to conduct the check cruise that would be more efficient for adjusting the original? Sure, and they are nothing new. One of the key points is point/plot selection. Since you have the benefit of the original cruise, a sorted list sample (discussed in Issue 40, page 1) can be employed to design your sample. This will insure that high volume/value points are selected with greater frequency and that all points in the original sample are up for consideration. The sorted list method also gives you a straightforward method for making the adjustment - it is simply the fully expanded estimate of the tract.
Although there is no underlying reason to, you may not want to apply your adjusted estimate to the original cruise. If the check cruise results are that the original cruise was within acceptable standards, you may want to avoid the possible confusion and work involved in changing the "answer."
Check cruising will be successful if you pay attention to the motivation for conducting the cruise and what information you will need to react to the data collected. Don't forget that the results of a check cruise can be used to reward your cruising team for a job well done or training when the check results fall short of your goals.
Originally published January 1999
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