Thanks Emeritus, OvertHawk and Duchess_Driver.
The three DME reports that I'm studying were all outbound and were apparently transmitted at the initiative of the flight crew. All three reports are preserved in two documents:
(1) document 2013.5.23.1: “Digitized notes from Northwest Airlines executive George Harrison, who was present at Seattle-Tacoma International Airport during the hijacking of Northwest Airlines flight 305”,
(2) document 2013.5.23.4: “curator report for examination of Aeronautical Radio, Incorporated (ARINC) teletypewriter printout documenting communications between the flight crew of Northwest Airlines 305 and Seattle-Tacoma International Airport” [which apparently refers to Northwest flight operations at SeaTac, not ATC, since ATC transcripts have no record of these transmissions],
both documents held by the Washington State Historical Society.
The reports were as follows:
"0340 - 7000' 14DME V23" (handwritten note at 7:40 PM PST);
“305 OUT SEA 14 MILES ON V23 OUT SEA” (between time stamps for 7:34 and 7:42 PM PST)
"0344 - 7000' 19DME V23" (handwritten note at 7:44 PM PST);
“305 STILL AT 7THSD FT 19 DME S V23“ (between time stamps for 7:42 and 7:45 PM PST)
"0418 23 DME PDX 100/" (handwritten note at 8:18 PM PST);
“0422 23 DME S of PDX (DME 10,000) 165 IAS” (handwritten note at 8:22 pm PST);
no corresponding message on teletype tape.
Reconciling these numbers (if taken at face value) yields some implausible ground speeds (or implausible winds). For example position 1 to position 2 = 5 nm in 4 minutes = ground speed 75 knots. If 14 were 13.5 rounded up and 19 were 19.9 rounded down, the ground speed would be 96 knots, which is still hard to reconcile with a reported IAS of 160 knots. Hence my interest in establishing the uncertainties and/or rounding practice in the DME reports, and in identifying other sources of error.
I’d be grateful for any professional input as to how to reconcile these numbers.