This explains the pheno hunting well.. a post I found from l33t, copied & pasted
Well its not that shocking cause when you 'hear 1 in 10 is a keeper' , it means that 1 in 10 will be satisfactory in all areas , smoke effect but growth and flowering times as well .On top of that its the yield and so on..So 1 in 10 doesn't mean only 1 in 10 is good smoke or a good plant but 1 will be 'better' and more 'suitable' to keep than the rest . For instance shanti says discard anything that goes beyond 14-16 weeks , but this doesnt mean that the slower ones are less good smoke wise.. its just for practical reasons you need to discard them as the return is relatively low . Now with SSH things are different almost all plants are keepers , you can find fast 8-9 weekers , skunk dominant plants or Hazyt plants , plenty good plants to choose from in a pack. Even the 'worst' SSH f1 is a good plant in general , and can easily compete many from the strains out there but with the Nevs the 'worse' plant is quite impractical and thus you need to be more strict when selecting. The 25% you refer to I think goes to fast finishers' ratio. Like shanti says only 1/4 (25%) of the f1 plants will finish before 14-16wks and it is recommended to select from those fast ones.
Things are relative as you say , even the Nevs which is from the least stable MNS hybrids , is still quite stable if you think about there is a keeper in every pack. Now what is a 'keeper'? Well it depends on what you look for and its subjective . Someone may care for yields more than quality or simply doing a different compromise . But there are objective reasons why one would cull a plant. Like plant resistance/yield/vigor/flower fastness etc
If you compare say mns SSH to StrainX from Xseeds , which sucks , then perhaps all the SSH F1's could be considered as Keepers simply cause they are way better than strainX.. But if you grow 100 SSH F1 you can easily see that some plants are just better than others. After the initial selection what comes to play is personal taste. Some will select a plant over a different one just cause they liked the smell better etc But if you grow enough strains and large numbers with time you get a feeling on how strict you must be during the selection process. It all heavily depends on numbers.
