Final answer:
A study with a ±3 percentage points error range is the most reliable. The margin of error represents the accuracy of the study's estimate. To improve reliability, increasing the sample size is an effective method.
Step-by-step explanation:
Among the given options for error ranges in a study, a study with a ±3 percentage points error range would be considered the most reliable. The stated ±3 percent error range is known as the margin of error, and it represents the range within which the true population parameter is expected to lie with a certain level of confidence. The margin of error is a reflection of the sampling error, which can be decreased by increasing the sample size. If the firm wanted to keep the error bound the same but increase their level of confidence, they would need to increase the sample size. Conversely, if the firm surveyed only 49 people while keeping the margin of error constant, the level of confidence would decrease because a smaller sample size increases the variability of results, making it less likely that they reflect the true population parameter accurately.