Scientific: hypotheses, theories and laws. Epistemological statements, carefully laid out, supported by facts but without the comfort of certainty.

The opposite of #3; "I want children" could be a desire :-)

A lot of people use imperatives as statements. "You *are* in favor of X" or "You *will* do Y"

This is usually in attempts at intimidation or indoctrination.

As such they are statements and are not members of any of your four listed classes.

I was taught that there are only two classes of statement- facts (which are statements that can be objectively proven or disproven- a fact doesn't have to be true to be a fact) and opinions (statements which can't be objectively proven).

Your examples 1, 3 and 4 are all facts- that is, they are disprovable statements. (If I have a copy of the US Almanac, or if I have knowledge that you've stated a wish for children elsewhere recently, or if I insert a Hurt-o-Meter into your skull and measure the results of pro-child sniping on your psyche, I can theoretically determine if the statements are true or false- hence, factual, either way.)

Thus: objective statistics and facts are factual; statements of a particular person's preferences and emotions are factual. In order to be an opinion, the statement must either be non-falsifiable or moot (debatable with equally valid arguments on all sides).

Value judgments are, without exception, opinions: "Drugs are bad," "Children are annoying," "Christmas is humbug, and any jackanape who thinks otherwise should be boiled in his own pudding," etc.

Statements of apparent fact where a category or qualification is ill-defined are opinions, as per #2 above. (Define "overpopulated"; does it refer to population density, sustainability, global carrying capacity of the environment, presence of undesired-by-speaker segments of population, etc. etc. etc.?)

As a general rule, my experience is that opinions are entirely and wholly emotional; we decide our position, and then look for facts to support our position. An opinion that originally derives from facts is a damn rare creature.

Statements of apparent fact where a category or qualification is ill-defined are opinions, as per #2 above.

And on the flip side of the same coin, statements of fact with the words "In my opinion" grafted on the front are still statements of fact, not opinion ("In my opinion, the Empire State Building is the world's largest building," "In my opinion, Jackson Pollak sold more paintings than Salvadore Dali").

There are as many classes as you want. You can make as few as you want by being general, and as many as you want by getting more specific. As a language nerd/professional, I could probably come up with a dozen classes of some sort just looking flipping through an old textbook or looking through some of the receptive/expressive language goals I write for my kids at work.

Theories, which are supported by facts (one hopes) and usually make checkable predictions.

Models or systems, which are similar to both theories and facts.

Plans: "I am going to get my tubes tied" or "my partner and I are going to start trying to conceive in July" are plans. Some plans are more nebulous than others: note that the first hypothetical plan has no timeframe attached.

Hmmm... I'm not sure where a statement such as "I believe in life after death" would fall. I guess it depends on whether you put the emphasis on "I believe", which is factual, or "life after death", which is a matter of opinion.

A major category is definitions, which can be argued as separate things but basically need to be accepted as postulates for given discussions.