Several source books are available to provide more comprehensive lists and explanations. The American Heritage Dictionary, 2nd College ed. Use an before any word beginning with a vowel sound, regardless of whether the word begins with a vowel.
Precision is the agreement among repeated measurements of the same quantity.
A sentence can be accurate but not precise . To effect is to bring about. An effect is a result or an influence. Ingesting alcohol affects her driving competence. An alternate is a substitute or second choice.