Check Cruising

   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
   Whether you run your own cruising department or use contractors, good quality control requires you to know if procedures are being followed and whether measurements are being taken within acceptable error limits. First, it's critical to distinguish between sampling error and measurement error. Sampling error arises from the fact that our sample looks at a subset of the population and that individuals within the population differ from one another. Measurement error is independent (usually) of sampling error and derives from imprecision of the instruments used to take measurements, personal errors of the cruiser, or improper application of field procedures.

   To assess the quality of the measurements being taken, some obvious prerequisites come to mind:

  1. The acceptable error limits for each measurement need to be documented and must be achievable given the equipment and time constraints placed on the field crew. Tightening (or creating) standards after the fact does not serve to gauge the quality of the crew's efforts. Similarly, it does no good to require measurement precision that the equipment issued to the crew cannot achieve. For instance, requiring heights to be within 1% for all but very short trees using a clinometer or Relaskop is not reasonable.
  2. The trees measured by the crew need to be locatable and identifiable by the check cruisers. Good flagging or other monument of point locations or plot boundaries is critical, as well as a documented order of measurement at each point or plot. For example always recording in-trees in a clockwise direction starting in the direction of travel.
  3. The equipment used to conduct the original cruise should be used to check the cruise. If instrumentation is changed between the original cruise and the check, any discrepancy between measurements will always be clouded with question of whether the instrument difference accounts for the error. An example would be comparing diameter measurements taken with a diameter tape with those from a caliper. New or different equipment may be useful in checking that the crew properly used the equipment issued to them or whether the equipment they used was calibrated properly.
  4. Care must be taken to obtain error-free measurements. It is reasonable and prudent to expend extra time and effort to obtain the check data.

If you find equipment out of calibration, you are faced with accounting for the error in all your check measurements and adjusting the original cruise for the differences if this is possible.

   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
   One of the important questions here is should there be a re-cruise (or should the contract payment be made)? All the considerations under assessing field procedures apply in this case as well. However, when have you found enough evidence to reject the cruise?

   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.

Cruise Adjustment
   After you have conducted a statistically valid sample of the original cruise, have measured check-sample trees carefully using your documented procedures, you have the basis for adjusting your original cruise if necessary.

   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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