Why were only three known-negative soil samples used to determine the Viking labeled release experiments' false positive rate?

According to

Levin, Gilbert V., and Patricia Ann Straat. "The case for extant life on Mars and its possible detection by the Viking labeled release experiment." Astrobiology 16.10 (2016): 798-810.

Three naturally sterile soils (Moon, Surtsey, and one Antarctic sample) that tested negative (i.e., the active and control sequences were essentially the same) showed the validity of the LR [Labeled Release] in not giving false positives (Levin and Straat, 1976b).

The referenced 1976 paper agrees: during the design and testing phases of the labeled release experiments (so before it was done on Mars), the false positive rate of the LR experiment was tested on only three known-negative soils. (There is also no mention of these tests being blinded, as far as I can tell the experimenters were aware of which soils were known-negative.)

Using naive statistics, that gives an expected false positive rate of $$1-\frac{3+1}{3+2}=\frac{1}{5}=20\%$$.

Why did the designers of the LR experiment, knowing that they would have to defend any positive result against accusations of being a false positive, choose to study the false positive rate with so few known-negative soils?

• My "flip a coin" methodology has a 1/8 chance of passing those control samples... – SE - stop firing the good guys Oct 4 '20 at 10:45
• Because there were more variables than a simple binary result? Storage at room temperatures for 4 months wouldn't have changed any non-life-related results, but could have affected life-related results - and it did. Sterilization likely wouldn't affect non-life-related results - but it did. Multiple different aspects were tested, so even with a few samples, you can discriminate more. If you get positive and negative results, you can correlate. – IronEagle Oct 5 '20 at 4:58
• @IronEagle and yet, a majority of exobiologists dismissed the results as false positives, a fact which Levin and Straat knew when they wrote the quoted paragraph in 2016. – Wouter Oct 5 '20 at 6:05
• The rule of succession only applies to where you do something, and the same thing keeps happening. There were at least 15 other soils tested, and multiple replicates leading to "thousands of tests". – IronEagle Oct 5 '20 at 18:36