EXECUTIVE SUMMARY 



Sampling criteria have been developed that can be used to plan and 

 execute underwater inspections of waterfront structures. The sampling 

 criteria are based upon statistical sampling techniques and the results 

 of underwater inspections of timber piers at the St. Helena facility in 

 Norfolk, VA, and steel sheet pile wharves at Mayport, FL. 



The methods currently used to plan an underwater inspection usually 

 involve the specification of the number of elements to be sampled as a 

 gross percentage of the number of elements in the facility (e.g., 10 to 

 20 percent). The actual selection of the elements is usually determined 

 by expert judgment of "typical" elements or groups. Since a substantial 

 part of the inspection cost is determined by the number of elements 

 inspected for each level of inspection, it is beneficial to minimize the 

 number of elements inspected for a desired level of confidence and pre- 

 cision in the inferences made from the data collected. 



There are two primary methods of obtaining the sample data. One is 

 a judgment sample and the other is a statistical sample. In judgment 

 sampling, a subjective decision is made regarding the criteria for 

 selecting samples from a population. This is commonly used whenever 

 there is expert knowledge about the population being sampled and the 

 investigator feels "comfortable" with the choice made. The disadvantages 

 of this method of sampling are : 



(1) The variability of the samples is unknown or nonquantif iable. 

 That is, the samples selected are based upon the inspector's 

 own biases. 



(2) No statistical model or method of estimating the population 

 parameters exists from the sample statistics because of the 

 bias in sample selection. 



(3) The smallest sample size for a given precision cannot be 

 determined since no probabilistic model exists for a 

 subjective sample. 



(4) The level of confidence for a given sample size cannot be 

 determined; therefore, the risk of being wrong with the 

 estimates cannot be specified. 



In the second sampling technique, called statistical sampling, 

 the sample is selected using probability (scientific) sampling. The 

 advantage of statistical sampling is that it permits the objective 



