Certain criminal cases are tried before juries. In these cases, jury members are tasked with very specific duties. According to the Preliminary Criminal Instructions, available through the Arizona Bar Association’s website, juries are responsible for deciding the facts and applying laws. These determinations must be made purely from the evidence presented by attorneys during the trial.
Speculations, prejudices and guesses are not allowed to sway these decisions. However, there are some situations in which juries must use their own judgment to determine the validity of evidence.
Jurors may be presented with testimony from experts and eyewitnesses. While juries are not allowed to let sympathy or prejudice factor into their decisions, jury members may need to weigh the credibility of witnesses.
Juries may also be asked to review circumstantial evidence. This is defined as evidence that does not specifically lay out a fact, but may indicate the existence of a fact. Here is an example of circumstantial evidence. Prosecutors in a robbery case call a witness. This witness states she saw the defendant enter a store. She adds that she heard a scream and saw the defendant run back out, holding a gun. When presented with this circumstantial evidence, jurors must determine whether they can infer that the defendant robbed the store.
In addition to making factual determinations, jury members are also tasked with applications of laws. However, juries are required to adhere strictly to the definitions and parameters expressed in the law. Members are not allowed to deviate from the law based on sympathy or disagreement with the outcome.