Making decision is not simple. Decision soon will be acted and then it will be impossible to change it. That might be one of the reasons why sometimes people have hard times making a decision. One needs to take in mind every single detail, choose one thing over other things, to foresee his or her own system of values and also the situation he or she is in. Despite of doing all of this we would never know for 100% if the decision will or will not be right.
In psychology there is lots of phenomenon cased that have influence on making decisions. I want to talk about two of them - deficit and conformism.

Deficit takes place when people are trying to take control over something very rare, unique. For example at the auction they sell unique rare things and buyers are paying lots of money to owe those things. If there were much more than for example just two or more paintings of a random painter, than the price would have become less. Therefore the desire from people to get that sting would decrease. Deficit would not have an influence on making decision anymore.
Besides deficit I want to say several words about conformism. Conformism is when one acts like majority of people. If one knows that 80% of population is going to vote for the candidate X, and he or she does not have her or his favorite candidate and he decides to vote for whoever is voted by the majority - that is conformism.

In addition, decision must provide situation. Regarding the situation decision may change. There are lots of words which can describe a situation, but I will stick with two opposite adjectives: certain and uncertain situations. Certain is a situation when one knows every detail of it, when everything is clear and there is no doubts that something is hidden. Uncertain is a situation, when one has no information about details, part of the information or all of it is not clear ad there are lots of doubts.

In my opinion the effectiveness of the weapons of influence - deficit and conformism depends on the type of situation - whether it is certain or uncertain. In other words, receiving the influence of deficit or conformism depends on whether situation is certain or uncertain.
Let's imagine a certain situation, when one has to choose just one pair of shoes from two of them, he or she likes both of the pairs, their prices are almost the same (so that difference between their values does not matter for that person), but the person who sells these shoes wants to sell one particular pair, what he or she needs to say to win over the costumer - make him or her buy what seller wants? Will it influence buyer if seller uses deficit influence and says: "those are the only ones on the market, the second pair is also good, but you can buy them almost on every shop". Here seller underlines that the pair of shoes is unique, rare. Another option for seller is to use an influence of conformism and say: "These are both great pairs of shoes, they are both equal, but everybody is buying the first one, the first one is really on demand". Which one seems more powerful for influencing on decision making?
In my mind, the effectiveness of weapons of influence changes throughout the uncertain situation. Now imagine that you are still buying a pair of shoes, but this time you can't see the shoes before you pay. You can not see details and the only thing you are depending on making decision is what seller says. Which would be the best to receive - deficit or conformism?

About Author / Additional Info:
I study Psychology at Ivane Javakhishvili Tbilisi State University