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Sources of Variability
There are many sources of variability inherent in lettuce seed bioassays. The following table outlines a few of these sources and how they might affect your experiments.

Potential Sources of Variability

Questions to Ask

Points to Consider

Viability (health) of the seeds
What percentage of the seeds will sprout under ideal conditions? If fewer than 80% sprout in your control, you may have a problem with your seeds or growing conditions.
 Definition of "germination"
Does everyone agree on how germination is defined? If a seed sprouts but has no distinct radicle that can be measured, do you count it as having germinated or not? This is the kind of decision that each experimenter can make, but in order to compare the results of your experiments with those of other scientists, you want to clearly state what decisions you have made.
Precision of measurement
If several people measure the same radicle, do they come up with exactly the same measurement? Is there greater variability in the data if several people take the measurements, compared with having them all done by one person? Judgement is often important in scientific measurements. At what exact point do you start measuring the radicle, and how tightly do you stretch it while measuring its length? Would everyone make the same decisions? Would you do it the same way every time?
Are you tempted to choose the best looking seeds for your control, and smaller seeds for treatments that you don't expect to grow much anyway? If you knew you would get a higher grade if your data indicated clear differences between the control and the treatments, would this affect how you selected which seeds to put in each dish? Scientists continually have to be on the lookout for sources of bias in their experiments. They are much more likely to get their results published and to continue receiving funding if they get good results from their experiments. Why do you think scientists might not want to be biased, in spite of these pressures?



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