An observational study determined that there is a strong correlation between getting less than 8 hours of sleep a day and lower test scores. Can it be determined that the low test scores are caused by sleep deprivation? Explain.

1. according to the internal revenue service, the mean tax refund for the year 2007 was $2,708. assume the standard deviation is $650 and that the amounts refunded follow a normal probability distribution. a. what percent of the refunds are more than $3,000? b. what percent of the refunds are more than $3,000 but less than $4,000? c. what percent of the refunds are less than $2,000?