Stats With Doug Flashcards
Are all populations normal?
No, not perfectly, but it has been accepted that the normal curve can be used.
What defines normal?
Bell shaped
Hump over mean
What does a flatter curve indicate?
More variable population
The more variable a population…
The bigger the sample size needed to be confident about statistical inferences
What does standard deviation and coefficient of variation imply
How much variance there are in the means of a population. Small in uniform populations.
What are some shapes that data can take?
Skewed
Bimodal
Strange
Bias
Systematic distortion where the value is either consistently too big or small.
Can be due to inaccuracies in equipment, improper measurements and statistical inaccuracies generated by survey.
Unbiased
Long run avg from repeated samples
Reasons for bias
Measuring bias
Sample selection (moving plots from a grid)
Calculation / weighting
What is the inflection point ?
The point on a normal distribution curve where horizontal distance becomes greater or less than vertical.
Within the inflection point will be 68% of the variation
If a population is large, chances are it will be
More variable, and require more plots to get confident data.
What are the units we use in surveying?
Stems per hectare
What are the magic z numbers for 95% and 90% confidence
95%: 1.960
90%: 1.645
What is the benefit of stratification?
Significantly reduce variation
What is the t table used for?
To increase sampling precision from 68% to any desired precision
