When bidding, I evaluate the probability of taking a trick based on the value of the card and how many I have in that suit. The higher the probability of taking a card, the more likely I am to bid on that card. For example: If I have the Ace of hearts, there is a very good chance that if I bid one, I will make that bid. Now, as I move down in card value to the King, the probability of me bidding on the King and making my bid becomes less likely than if I had the Ace.
Throw in another variable, that being the number of cards you have in a certain suit. The more you have of a suit, the weaker or less probable you will be in bidding and taking a trick. For example purposes, let's say that you have all 13 hearts. What would you bid? Since this is a partner game, I would bid nil: the only way you would not make the nil would be if your partner was void of spades and you had the lead after the bidding was finished. The chances of that happening are slim to none. Take the same scenario but let's say you have 12 hearts. If you bid nil, you are less likely to get it than if you had all 13. You can take other scenarios based on the facts and figure the probability on your own. There are many strategies when bidding, but the majority are based on probability.
One additional thought when bidding: the situation dictates how conservative you want to be. For example purposes, let's say that your team is within 20 points of winning, your bags are 4, and your opponents are 200 points behind you. It is best to be conservative and not reach with your bid nor risk bidding a nil. Go for the win in the most probable way based on the total strength of your hand. A lot more can be discussed and debated on bidding strategies. I hope that the above has provided some food for thought.