I can not for the life of me remember who posted this, nor can I find the threads. But this member had a great system for sorting out the key dates. This is the original post that I copied for reference...

My little project for the day. This may have been done before, but I'd like to present my findings. I looked at mintage figures for the Lincoln cents 1909-2010. I divided them into the two traditional groups: wheats (1909-1958), and modern (1959-2010). I am not including varieties such as 1955 DD, 1960 small date, etc., since these have no specific mintage figures.

I took the log (base 10) of the mintage figures and determined their statistical z-score. (The log value gave, in my opinion, more realistic z-score distributions than using the raw mintage figures.) The z-score is a measure that tells how many standard deviations it is above or below the mean. Key dates would have very negative scores. What I was trying to do is to see what z-scores are for the traditional key dates and then compare these values to the more modern cents.

Here are the results for the

LWC:

1909 S VDB -3.07 key

1931 S -2.71 key

1914 D -2.51 key

1909 S -2.25 key

1924 D -2.05 semi-key

1911 S -1.76 semi-key

1914 S -1.74 semi-key

1912 S -1.70 semi-key

1931 D -1.70 semi-key

1926 S -1.69 semi-key

1915 S -1.65 semi-key

1910 S -1.51 semi-key

1913 S -1.50 semi-key

I used the definitions of key and semi-key from

http://www.lincolncentresource.com/index.htmlUsing the cutoff value for a key date to be -2.2 or smaller and a semi-key between -2.2 and -1.5, I then determined the following for the modern cents and used this definition to label them as key or semi-key:

(1959-2010):

1968 S -3.12 key

1973 S -2.87 key

1972 S -2.65 key

1974 S -2.55 key

1971 S -2.24 key

1969 S -2.20 key

1960 -2.10 semi-key

1962 -2.06 semi-key

1959 -2.06 semi-key

1970 S -1.90 semi-key

1961 -1.79 semi-key

1963 -1.79 semi-key

It is interesting to note that WITHIN these groups, the 1968 S is comparable to the 1909 S VDB! You can make similar comparisons between the groups, such as the 1974 S is very comparable to the 1914 D.

I know this is a long read and there are many interpretations, but I thought this was interesting. I also felt is was important to have the two groups of data span about the same number of years. I also know there are other variables to consider to determine what is a key date.

Finally, there has been talk about how low mintage the 2009 cents have been. Here are the results of these:

2009 P -1.32

This compares best with:

1923 S -1.29

2009 D -1.17

This compares best with:

1932 D -1.17

Here's the 2009 breakdown:

2009 P4 -3.05

2009 D4 -2.63

2009 P1 -2.26

2009 P3 -2.16

2009 D3 -2.10

2009 D1 -2.05

2009 D2 -2.02

2009 P2 -1.98

He followed that with this for Jeffersons...

Here are the results for the 1938-1963

Jefferson nickels (sorted by z-value). I have listed the "top 10":

1950-D -2.49

1939-D -2.24

1938-S -2.10

1938-D -1.85

1939-S -1.67

1951-S -1.52

1955 -1.47

1949-S -1.32

1950 -1.31

1948-S -1.19

Generally, the top 5 have been considered to be "key dates", but in comparison to the Lincoln wheat cents, perhaps only the top 3 are "key dates" whereas the 1938-D, 1939-S, and the 1951-S should be considered as "semi-keys". Just my opinion.

Here's where it gets interesting. With the more modern nickels (1964-2009) here are the results (again the "top 10"):

2009-P -3.58

2009-D -3.34

1968-D -2.33

1968-S -2.14

1971 -2.09

1967 -2.08

1969-S -1.88

1965 -1.72

1966 -1.52

1975 -1.29

Let me reiterate this...

THIS IS NOT MY WORK I am only posting what someone else did. Hopefully whoever this was will see this and I can make a note.