According to thepp.org, of approximately 11941 North American Tournament Scrabble words containing IE or EI, there are 2169 total exceptions.
Thus, to blindly follow the rule for ie and ei, we would get 80% on our spelling test if we could get everything else right.
As I was unable to find an in depth statistical study of the “i before e” mnemonic online, I was forced to analyze the data myself. Casually looking at the list, we see approximately 20% of the exceptions to the rule are actually made when we “change y to i and add ed/es/ent/etc”. For example decency becomes decencies in violation of the “i before e” rule. This accounts for nearly all “C then IE” exceptions, and leaves us to remember words like science, glacier and ancient. Also watch for adding ing to a word that ends in e such as being.
A further 40% of the exceptions can be eliminated with the additional “and when it’s said ay as in weigh or neighbor”. My conclusion is then that for 90 to 95 percent of words the rule with that extension allows us to correctly order i and e.
If a teacher considers the “i before e except after c, and in words when it’s said like ay as in weigh or neighbor” mnemonic unsatisfactory, what is he or she doing when it comes to spelling - rote memorization? In my opinion, pure rote memorization would lead to a class with a lower spelling test score than a class that learned this mnemonic rule. Therefore, despite its imperfection, I feel it serves its use.
For one last example, should I see the spelling formosafeids I will naturally consider the last syllable to have the ay sound. “I before E except after C and when it’s said with an ay sound as in weigh and neighbor” also conveys a phonic pattern that I acquired as a native English speaker.