Schemata and heuristics are used in uncertain or novel situations. A schema, which is the singular of schemata, is a mental structure that helps organize perceptions by comparing them to past events and filling in information gaps with statistically likely information. A heuristic is a mental process for making judgments based on insufficient or inaccurate information by relying on shortcuts and intelligent guesses.
Schemata and heuristics are used in different contexts. A schema is used to organize perceptions, while a heuristic is used to make judgments. A schema is therefore most often used in novel situations in which the mind is receiving perceptual stimuli faster than it can make sense of them and must take mental shortcuts to process the information. Heuristics are used in situations in which judgment is required without sufficient information to enable the individual to make a fully informed decision.
A major difference between schemata and heuristics is in how they are commonly used. Schemata are used to replace missing information with made-up facts that seem plausible or make plausible connections between independent events or stimuli. Because of this filling in the gaps, schemata may create false memories when recalling events for which memories are very sketchy, if the reality of a recalled event is different from schematic representation of the event. Heuristics, however, do not rely on the creation of false information, but instead, make a judgment by taking what information is readily available and extrapolating from it.
Another difference between schemata and heuristics is that schemata are used on a subconscious level. When recalling partially forgotten memories, people are incapable of separating recalled facts from schematic memories because the schema for memory exists on a subconscious level. For this reason, false memories are a major problem in courtroom testimonies -- witnesses can be led to testify falsely, without realizing that their statements are false. On the other hand, while the exact mechanism of a heuristic occurs subconsciously, people are generally aware of when they are forced to use a heuristic to make a judgment and will base their confidence in the judgment on that fact.