Bookworm has indicated the limitation of the mk 1 eyeball, stating on another thread that he has trouble seeing traffic, even when told about it. I must say I was surprised, but maybe it is endemic these days.
Then I wondered why. Glider pilots seem to have more success at seeing traffic. Generally I think that a small helo pilot sees more than what BW suggests and multi crewed aircraft have even more success.
Could it be that pilots who are flying aircraft with fewer instruments and fewer distractions in the cockpit have more success at spotting other traffic? Maybe the effectiveness of lookout is a factor of aircraft design, ie better chance in a Tommahawk than a Cherokee?
Interesting Topic for TonyR's second PhD??!!