6
Referent Cohort
GENERAL SCHEME
We decided on a two-stage scheme for selecting nonparticipant controls. For study personnel on ships, we began first by selecting a matching control ship for each participant ship. We then obtained the two quarterly rosters closest to, and bracketing, the dates of the corresponding test. Using these two quarterly rosters, we compiled a roster of all Navy personnel on the control ship who became part of the referent population for the study. In contrast to the process for identifying participants, we did not obtain and key information from the daily personnel diaries for the control ships. Although up to five Army light tugs, manned by Navy personnel, participated in several Project SHAD (Shipboard Hazard and Defense) tests, their complete personnel rosters were never found by the Department of Defense (DoD) or by us. Because there were relatively few Navy participants on these light tugs and because we had more than sufficient controls selected from other ships in the same tests, we did not select specific control ships for the Army light tugs.
Because we did not sample individuals from the control ships but instead took all persons on a selected ship (i.e., a census), our selection of controls is not strictly speaking a cluster sample. There is, nonetheless, an unmeasured component of variability associated with the sampling of ships. To properly estimate this component of variability would have required a much larger sample of ships than we had.
The process for Marine control units was similar in that each Marine participant unit was matched with a control unit. Finally, because there were so few identified Army, Air Force, and Coast Guard participants and controls (N = 160), we omitted them from most of the analyses.
DETAILED INFORMATION
Stage 1:
Selecting Ships
DoD personnel provided us a list of potential control ships for each participant ship and test, choosing potential control ships of the same type and class. We then developed formal control ship selection criteria that were sent to our expert advisory panel and to members of the Vietnam Veterans of America’s (VVA’s) Project SHAD Task Force for comment. The final control ship selection criteria are shown in Table 6-1. In general terms, we selected control ships to be the same type and class as the corresponding participant ship. We further selected control ships
TABLE 6-1 The Process of Selecting Control Ships
Step |
Procedure |
Review the DoD control ships list. |
Verify similarity of ship type (the control ship should be of the same type and class as the test ship, or a similar type and class). Determine size of the complement (the complement of the control ship should be at least as large as that of the test ship, or larger). Determine operating area (the control ship should have operated in the same ocean area as the test ship [in most cases, the Pacific Ocean]). Determine operational status (the control ship should have been in an operational status during the test period, meaning it should not have been in an extended overhaul or dry dock status). Determine home port (the control ship should have had the same home port). |
Designate potential control ships. |
Remove from consideration ships that do not meet all the criteria (except for home port).a |
Rank candidates. |
Assign a rank to each potential control ship depending on the similarity of control ship to participant ship (ties are allowed). |
Select the control ship.b |
Take the control ship with the highest rank; if there are ties, select randomly. |
aDeviations from these criteria were avoided when possible, but could be necessitated by factors that prevailed at the time of the tests; e.g., there may have been no similar type ship in the DoD list of potential control ships, all DoD-listed potential control ships may have been in a nonoperational status during the exact period of a test, or the potential control ships may not have had the same home port as the test ship. bA test ship that served in multiple tests may have more than one control ship selected because of changes in a potential control ship’s fulfillment of one or more of the above criteria; e.g., it may have been operational during the period of one test, but in dry dock during another test. |
with a complement, operating area, and home port similar to that of a Project SHAD vessel. When there were multiple possible control ships, we ranked them in order of desirability and selected the closest match. To assist in characterizing potential control ships, we hired Jim Quinn, Commander, USN retired, as a consultant. For the few Marine units, we selected a similar unit in operation at the same time. In general, we tried to select the identical unit in a parallel battalion or division. The final list of control units is shown in Table 6-2.
Stage 2:
Selecting Individual Subjects
Once a control ship had been selected, we used a similar process as was used for participant ships; that is, we obtained the quarterly BuPers reports for the corresponding time periods. However, we did not make use of personnel diaries. We keyed the entries from unit records and produced a list of control subjects, identified by name, military service number, and rate or job title (for officers). The process for Marine control units was similar to that for Marine participants, except that, again, we did not use personnel diaries. In some cases, we selected Marine nonparticipants from the same unit as the participants. We do not think that omitting potential controls who would have been identified solely from personnel diaries is a substantial omission; only 5 percent of Navy participants were identified solely from personnel diaries.
In preparing a participant roster for their own purposes, the DoD supplemented the unit record information on participants with other data from other sources, for example, Social Security numbers (SSNs) from individual personnel records, addresses from other sources, and so on. Because the DoD did not assemble a control roster, we had to begin ab initio from unit records to assemble the control roster for our study, and our controls never had initial identifying information beyond name, service number, and rate. Although we undertook a similar process as the DoD, we began our identification of controls later, and thus our control subjects were typically less well identified than participants and harder to trace, locate, and contact.
TABLE 6-2 List of Participant and Control Units Showing Selected Characteristics
Participant Unit |
Control Units |
||||
Test Name and Unit Name |
Ship Type |
Operating Area |
Unit Name |
Ship Type |
Operating Area |
Autumn Gold |
|||||
|
Auxiliary ship |
Pearl Harbor |
|
Radar picket ship |
Picket Station 1 (Canada), Treasure Island, Portland |
|
Attack transport |
Pearl Harbor |
|
Attack transport |
Long Beach |
|
Tank landing ship |
Pearl Harbor |
|
Tank landing ship |
Japan, Philippines |
|
Destroyer |
Pearl Harbor |
|
Destroyer |
San Diego |
|
Guided missile destroyer |
Pearl Harbor |
|
Guided missile destroyer |
San Diego |
VMA 214, Marine Air Group 13 |
— |
— |
VMA 332, Marine Air Group 14 |
— |
— |
Big Tom |
|||||
|
Auxiliary ship |
Pearl Harbor |
|
Auxiliary ship |
Subic Bay, Philippines |
|
Submarine |
— |
|
Submarine |
San Clemente, San Diego |
Copper Head |
|||||
|
Destroyer |
— |
|
Destroyer |
Norfolk, Portsmouth |
DTC Test 69-10 |
|||||
|
Dock landing ship |
— |
|
Dock landing ship |
Little Creek, VA; Onslow Beach, SC; Morehead City, NC; Vieques |
Landing Force Carib 1-69/BLT 1/8 |
— |
— |
1st Battalion, 6th Marines, 2nd Marine Division |
— |
— |
VMA 324, MAG-32 |
— |
— |
VMA 324, MAG-32 |
— |
— |
DTC Test 69-31 |
|||||
|
Destroyer |
Pearl Harbor |
|
Destroyer |
San Diego |
DTC Test 69-32 |
|||||
|
Auxiliary ship |
Pearl Harbor |
|
Auxiliary ship |
South China Sea, Thailand, Vietnam, Subic Bay, Special Operations |
Eager Belle I |
|||||
|
Auxiliary ship |
Pearl Harbor and maneuvers |
|
Radar picket ship |
Picket Station 1, Picket Station 9, San Francisco |
Eager Belle II |
|||||
|
Auxiliary ship |
Pearl Harbor |
|
Radar picket ship |
San Francisco Picket Stations 1, 3, 9 |
Participant Unit |
Control Units |
||||
Test Name and Unit Name |
Ship Type |
Operating Area |
Unit Name |
Ship Type |
Operating Area |
|
Tank landing ship |
Pearl Harbor |
|
Tank landing ship |
Yokosuka, Kobe, Taiwan, Po Hong Do, Okinawa |
|
Destroyer |
Pearl Harbor |
|
Destroyer |
Subic Bay, Hong Kong, Manila, Yokosuka, Okinawa |
USS Granville S. Hall (YAG-40) |
Auxiliary ship |
— |
USS Interdictor (AGR-13) |
Radar picket ship |
— |
USS Navarro (APA-215) |
Attack transport |
— |
USS Noble (APA-218) |
Attack transport |
— |
Marine Medium Helicopter Squadron 161 |
— |
— |
Marine Medium Helicopter Squadron 161 |
— |
— |
Errand Boy |
|||||
|
Auxiliary ship |
Pearl Harbor |
|
Radar picket ship |
San Francisco, Picket Station 7 |
Fearless Johnny |
|||||
|
Auxiliary ship |
— |
|
Auxiliary ship |
Hong Kong, Subic Bay |
|
Auxiliary ship |
Pearl Harbor |
|
Auxiliary ship |
Hong Kong, Subic Bay |
Flower Drum I |
|||||
|
Auxiliary ship |
Pearl Harbor |
|
Radar picket ship |
San Francisco, San Diego, Radar Picket Stations 1, 5, 7 |
USS Granville S. Hall (YAG-40) |
Auxiliary ship |
— |
USS Interdictor (AGR-13) |
Radar picket ship |
— |
Folded Arrow |
|||||
|
Auxiliary Ship |
Pearl Harbor |
|
Auxiliary ship |
Subic Bay, South China Sea |
|
Submarine |
Pearl Harbor |
|
Submarine |
Subic Bay, Special Ops |
Half Note |
|||||
|
Auxiliary Ship |
— |
|
Auxiliary ship |
Taiwan, Hong Kong, Special Operations |
|
Auxiliary Ship |
Pearl Harbor |
|
Auxiliary ship |
Malaysia, Taiwan, Special Operations |
|
Submarine |
— |
|
Submarine |
Pearl Harbor, Subic Bay |
|
— |
— |
|
— |
— |
Participant Unit |
Control Units |
||||
Test Name and Unit Name |
Ship Type |
Operating Area |
Unit Name |
Ship Type |
Operating Area |
High Low |
|
— |
|
|
|
|
Destroyer |
|
|
Destroyer |
Hong Kong, Yokosuka, San Diego |
|
Destroyer |
— |
|
Destroyer |
San Diego |
|
Attack transport |
— |
|
Attack transport |
San Diego, Pearl Harbor, San Clemente |
|
Tank landing ship |
— |
|
Tank landing ship |
Numazu, Yokosuka, Okinawa |
Magic Sword |
|||||
|
Auxiliary ship |
Pearl Harbor |
|
Auxiliary ship |
Durban, Subic Bay |
Purple Sage |
|
— |
|
|
|
|
Destroyer |
|
|
Destroyer |
San Diego |
Scarlet Sage |
|
— |
|
|
|
|
Destroyer |
|
|
Destroyer |
San Diego, Long Beach |
Shady Grove |
|||||
|
Auxiliary ship |
Pearl Harbor |
|
Auxiliary ship |
Bremerton, Wash; Panama Canal Zone; Guantanamo Bay, Cuba; Portsmouth, VA |
|
— |
— |
|
— |
— |
|
— |
— |
|
— |
— |
|
— |
— |
|
— |
— |
|
— |
— |
|
— |
— |
|
— |
— |
|
— |
— |
Speckled Start |
|||||
|
Auxiliary ship |
Pearl Harbor |
|
Auxiliary ship |
Subic Bay, South China Sea |
*Original participant files contained personnel in these units who were subsequently removed from the study. |