The following is based upon a reply I made to a thread by “stevieamy” in the New Members Forum  entitled “Why?”, Stevie was basically asking why go all in as opposed to simply betting a substantial proportion of your stack instead.
Feedback was positive and I thought I’d re-write it and preserve it here on my Blog, any constructive critisism and comments are welcomed as are any additional advice on the issue that could be incorporated into a future edit.

In freerolls and micro stake MTT's large bets or raises often aren’t enough to deter a call from some of the loose, inexperienced or undisciplined players that you are likely to be encountering in the early stages of a tournament, the decision to go all in or not may have to be made quite frequently.
As you go deeper in the tournament you a likely to encounter players with extremley long stacks who will use their chip advantage to pressure the table not to mention short stacks fighting for their tournament life.
The all in, from a poker maths point of view, could be described in a lot of situations as an illiogical action in poker, it is often made with total disregard for hand and pot odds but on other factors that make up the game as compelling as it is.  Strictly speaking though, if hand and pot odds were the sole basis of making a decision to bet in poker there would be no calling or raising of intial blind bets in the vast majority of hands. Any initial call or raise is factoring in impied odds as well as a lot of the issues about to be mentioned, the all in just takes increases the importance of these other factors.
There are probably two primary purposes to an all in, firstly for the player making it to scare off all their opponents into folding so that they can take the pot without any further play, or secondly if called they hope to increase their chip stack significantly whilst potentially eliminating or chip crippling the calling opponent or opponents. Don’t forget that every single player eliminated from a tournament did so going all in, the tournament winner either never went all in or was fortunate enough to survive each time they did.
So its crunch time, here are a number of factors that I would consider when weighing up the decision to go all in, the importance and weighting of each factor within the decision will vary from situation to situation and as you progress deeper into the tournament .

You - Are you a loose or tight player? What is the range of hole cards you are personally comfortable going all in with at any given stage of a tournament?
You Tournament Objective– Are you killing a bit of time playing in the tournament or are you looking for a finish, if so how deep are you aiming? In the case of the PSO Skill League you may be simply aiming for a positive point score. More on this in “The Stage of the Tournament”.

The Purpose of Your All In – As mentioned in the introduction, ask yourself what do you want to achieve with your bet, sure you want to win the hand and increase your stack but are you looking to scare off the rest of the field with your bet to take the blinds and any previous bets that are in or are you looking for action in the hope of a major chip gain.

This in most instances will depend upon your current chip stack, a player with a long stack and in no risk of being hurt by a call is likely to extend their range of all in hands in an attempt to dominate the table, “the chip bully”, their all in will often threaten the tournament life of any would be callers whilst only being a minor inconvenience if the hand is lost. A short stacked player on the other hand is more likely to be going all in with a premium hand with the hope of being called by a long stack and doubling up.
You will often see a short stackers all in called by one or more players even though it is still a significant amount of chips. Often these calls are due to a false perception that the short stacked player must be weak player due to their lack of chips, part of the psychological aspect of poker, however multiple calls of a short stacks all in can be a tactical play pre-bubble.
I’m sure you will have seen a short stack go all in and be called by a number of larger stacks at the table, the flop, turn and river go un-bet  and the showdown is made. The following is a brief explanation of what is going on here, it is play that experienced players may do conciously and less experienced players find themselves involved in unconciously.
Lets say our short stack has 8BB of $100, we wouldn’t say they are extremely short stacked therefore their all in is likely to be based on premium hole cards AA, KK, QQ or AK, they have bet Under the Gun on a 9 seat table adding weight to our belief they have a monster, there a 3 players to be eliminated to the finish places, again would they risk their tournament life so close to a finish if they didn’t have that monster, we put them on AA, calling would appear a very risky move but eliminating the short stack would bring a guaranteed finish closer for all the other players at the table.
In our scenario the $800 to call isn’t going to put too big a dent in the majority of stacks at the table and certainly isn’t going to be a risk their tournament lives, the big risk is that the short stacks hand carrys resulting in a significant increase to their stack and a possible prolonging of the pre-bubble period. What is needed could be interpreted as a form of legal collusion, ideally a minimum of 3 players call the all in, regardless of their hole cards. Heads up against unknown hole cards our short stack has an 85% chance of winning with their AA, against 3 opponents that has dropped to 65%, 4 drops it to 56%. The callers then check the flop, turn and river, unless they hit the nuts and the showdown is played out.
Experienced players conciously avoid betting the flop turn and river as they know pushing one of the other players off will increase our short stacks odds of winning the hand that is unless they have the nuts in which case its open season. Inexperienced players may be avoiding betting as they have called in to the pot with a speculative and want to avoid committing any more chips unless absolutely necessary, they can often unwittingly destroy this play by aggressively betting a weak made hand and pushing the rest of the players off.
An analogy would be Lions attacking a Leopard, a solitary Lion has little chance off killing a Leopard but the more Lions the better the odds of them making the kill, sure only one Lion is going to get to feed in our analogy but another predator has been eliminated which is to the benefit of our Lion pack.
The Betting so far– Will the all in be the first action or has there been previous betting? Leading an all in and calling one should be viewed differently, your range for calling one is likely to be slightly tighter and will probably require consideration of more factors than when leading with one.
Is your all in against limpers or has somebody made a substantial raise beforehand indicating that they have a decent hand and are therefore more likely to call you, bear in mind that an all in following a raise is likely to scare off any speculative callers of the initial raise behind you.

The Blinds – Not really a factor as such, obviously an all in that is 30x the blind looks far more intimidating than one that is 3x, however if you only have 3 BB’s you're very short stacked and should be looking to go all in at the first opportunity.

Stacks on the Table - An all in is most likely to get action from long and short stacks as mentioned previously, take into account their positions relative to you and whether they have acted yet. Note that a very short stacked player is likely to call an all in with any Ace/rag, connectors or suited cards, especially if they are due the blinds soon.

Your own stack is naturally of importance, are you flexing your chip strength or in a state of chip anorexia. If you are very short stacked you may have little alternative than to push all in with the first half decent hole cards that come your way, best to go out on an all in of your own choice rather than the blinds forcing you all in.

The Players – Do you have a good read on them? Knowing who is tight and who is loose is an important piece of information, have you seen previous showdowns, particularly all ins, what sort of range were individual players prepared to bet on and especially go all in with? Try to get into the habit of making notes on players for reference in this and future tournaments.

Are you new to the table or are there new players at the table? An aspect of MTT play that can often throw a player off their stride is being moved to different tables, often a number of times in quick succession. This will limit your opportunity to get a read on the players and the table generally but similarly they will have had no opportunity to get a read on you. You may find that you are no longer the dominant chip holder at a new table and previous domination plays may no longer be as viable.
Your Image– Do your opponents view you as a nit or a maniac? What type of hands have you revealed at showdowns? If you have been involved in very few hands an all in should send a stronger message than it would if you’ve been involved in every other hand. Similarly if you’ve previously been seen chasing to showdown or betting with nothing your likely to be viewed as loose and attract a call, which may not be a bad thing if you do have a monster, reverse psychology at its best.

Your Table Position – It always seems to be that you’re dealt that premium hand when under the gun, the truth is this should be a good psychological position to go all in, it tells your opponents “I have a hand so strong I’m prepared to play it from bad position”, unfortunately it is likely that a fair number of your opponents will not be savvy enough to read that fact, table position to them may be whether it is near to a window or a door!

An all in from the cut off or button when everyone has folded to you will often be seen by the blinds as a pot steal and you may get a call from the blinds as your bet is perceived as simply a stealing bluff. Have you previously folded a number of hands in this situation? If so they may read you as having the hand you are saying you have. If it is that you have continuously raised before in this situation consider how often the blinds called you and with what.
Generally the earlier your position the tighter the range with which you should consider going all in. Under the gun you could be called by the entire table and be that Leopard suddenly surrounded by a pack of Lions.

The Stage of the Hand – Is it a pre-flop all in or is there community on the table, if so what was the betting on the previous rounds, including your own. How much has your hand strengthened and what potential strengthening of the opponents hand is there in that community?

A related consideration from flop onwards is if you are fortunate enough to have hit the nuts, particularly on the flop. Inexperienced players will often get a nose bleed and throw all their chips in immediately but consider whether an all in will scare off your potential customers? In most cases a resounding yes, better to low bet or even check and hope your opponents do the betting for you to keep them interested in the hand, you are disguising the strength of your hand rather than proclaiming it to optimise the pot value.
If you are lucky the community will give somebody else a strong made hand and then you may find yourself calling their all in in that glorious but rare position of knowing you have the pot in the bag.

The Stage of the Tourney – If it’s the very first hand of the tournament be very wary about pushing or calling an all in, sure double, tripling or even quadrupling up at this stage would be great but an all in on the very first hand is likely to attract a lot of fish in the water, the more there are in the lower your odds of carrying. I would advise similar caution throughout the early rounds, wait until you are settled and have a decent read on the players and table.

Pre-bubble will find a good proportion of players tightening up and less likely to risk their chance of a finish by leading or calling an all in and if you are fortunate to be long stacked at this stage you may find this a fruitful time. If you a medium or short stacked however you will need to consider if you are prepared to put your tournament life on the line with a finish so close.
Beware the long stacks that are already comfortably in a finish, they are probably on a feeding frenzy with all the tight play and their range will be wide and as such that they may see an all in as an opportunity to climb up another finishing position at the risk of climbing down one.

Post-bubble the play will usually become frantic with a mixture of long stackers using their chip advantage and short stackers throwing last ditch efforts to stay in the tournament. At this stage you have made a finish, the pressure is off, everything from here is a potential bonus, you will probably feel more relaxed about either throwing or calling an all in but so are your opponents and with blinds being so high at this point your stack and therefore your all in bet will need to be intimidating if your aim is simply to put off any potential callers.

Your Hole Cards– The basis upon which most all in decisions will be made but in a lot of instances they may not necessarily be the most important factor in choosing whether to go all in or not, sure they determine whether you win the hand or not but maybe not so much in whether you get to that stage in the first place.

Having taken all other factors into consideration look at your cards again and be aware of their limitations. I mention only the premium hands as in truth anything less in freeroll and micro play as a pre-flop all in hand is like playing with fire though if you’re very short stacked you will have no alternative to push with something more speculative at some point.

AA – no brainer in any situation, though consider this scenario, it’s the first hand of the tourney, 9 seats, you’re the BB, UG goes all in and everybody calls, what do you do? Do you trust hand odds or the law of sod? You have a 35% chance of winning if you were wondering.

AKs, AKo – remember you’re already down to any pair, if you’re calling an all in chances are you’re already playing catch up and need to improve.

KK, QQ – A/rag is the usual killer here and unfortunately in freerolls and micro a lot of players get a nosebleed with A/anything and will call you. If the flops already out and showing an A and you’re not already on a set think twice about calling or pushing an all in.

Approx hand odds of these hands winning against unknown opponent hands pre-flop. (I used an odds calculator so apologys if the percentages are out).

Against 1 opponent

AA           85%
KK           82%
QQ          80%
AKs         66%
AKo         65%

Against 2 opponents

AA           73%
KK           69%
QQ          65%
AKs         50%
AKo         47%

Against 3 opponents

AA           64%
KK           58%
QQ          53%
AKs         41%
AKo         38%

Hand odds are very much worth studying and if possible learning, in conjunction with pot odds their importance in making sound judgements and playing solid poker cannot be emphasised enough. At this point it is may be worth considering that pre-flop 7,2o has a 32% chance of winning against 1 opponent with unknown hole cards, so when that large stack is prepared to risk a 3rd of their chips or less to call your pre-flop all in with their 7,2 consider that their play may be a bit more well thought out than it simply being a “donk” call.
Finally a note on bluffing an all in? Yep rumour has it that players sometimes pretend to have cards they don’t actually have when they play poker and in freerolls and micro play there’s a lot of it. If you are bluffing all the other factors mentioned will become far more important. Remember though that as there is often a lot of bluffing in freerolls (for many players it’s the only weapon in their armoury) players will often perceive large raises and all ins as bluffs, distrusting souls. My advice, never bluff an all in unless you can afford to lose the hand. Similarly don't be tempted to call an all-in based solely on the belief that it is a bluff, chances are it isn't.

If at the end of the day you have weighed up all the factors, gone all in with your AA, AK’s or whatever and end up being called and turned over by a speculative hand that strengthens console yourself in the fact that you have played the hand based on sound judgement and in the long run sound judgement will always triumph over no judgement and blind luck.