P

parameter, property--A parameter is an arbitrary constant. A
property is an explicit value or characteristic.

The parameters for the first test were 6 to 12 V.
Four parameters, three in space and one in time, are needed to specify an
event. One of the most important properties of iodine is its low
temperature of sublimation.

percent, percentage--Percent is colloquial when used as a
substitute for percentage. Percent is always specific and should
be used with a number (10 percent), whereas percentage is not
specific and is never used with a number.

A large percentage of the reports were
finished.

A small percentage of the funding is
available.

practical, practicable--Practical means "useful in actual
practice." Practicable means "capable of being put into practice."

precision -- See accurate.

predominant, predominate--Predominant is an adjective
describing something that has the greatest influence or is a prevalent factor.
Predominate is a verb, and means "to be the most important" or "to have a
controlling influence."

The predominant crop on the island was sugar.
Sugar processing plants predominated the local economy.

preventative, preventive--Preventive is preferred.

principal, principle--As a noun, principal means "head" or
"chief;" as an adjective, "highest" or "best." Principle means "basic
truth, law, or assumption." Principle is NEVER an adjective.

The principal author gave the lecture. He acts
according to the highest principles.

prior, previous, before--Prior is an adjective and is
interchangeable with previous. Don't use prior to. Before
is an adverb.

He was hired according to prior agreement. The
agreement was reached before his arrival.

proved, proven--Proved is preferred as a verb. Proven is
better used as an adjective.

He has proved his point. He has a proven
record of achievement.

providing, provided, if--Do not use providing for either
provided or if.

Providing jobs is difficult now but will be
easier provided (if) next year's budget is adequate.