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About the ACT
The ACT sports four trademark multiple-choice subject tests covering English, Math, Reading, and Science. These are designed to evaluate your overall educational development and your ability to complete college-level work. You'll have 2 hours and 55 minutes of dedicated test time to complete the subject tests, not including breaks.
As far as scoring goes, your subject test scores (ranging from 1 to 36) are determined after throwing out any incorrect answers - only correct responses count! The four areas are then averaged together to come up with your overall, or composite, score.
The ACT also includes an optional 30-minute writing test designed to measure your skill in planning and writing a short essay. This segment is your chance to highlight your writing skills! If you opt to take it, the additional scores will be reported, along with comments about your essay. These scores are reported separately.
So, if writing is a weak area, you might want to take the ACT and skip the writing section, since it's currently optional (although some schools require it). If writing is your strength, having extra kudos passed on to your choice schools may benefit you.
About the SAT
When looking at the SAT in comparison to the ACT, a clear difference is that the SAT is designed to evaluate your general thinking and problem-solving abilities. It kicks things off with a required 25-minute essay. This is the start to the Writing section, which you'll complete in addition to the Critical Reading and Math sections. The SAT differs from the ACT in terms of the amount of time you'll have to complete it (3 hours and 5 minutes) and the format in which you provide your answers.
Similar to the ACT, the SAT has multiple-choice areas, but it also has a part in the Math section where you'll be required produce your answers - no chance of guessing from a set of choices here! And unlike the ACT, the SAT doles out a slight penalty for wrong answers on the multiple choice questions (but not on the student-produced ones).
When considering the ACT vs. the SAT, keep in mind that both tests allot ample time for completion, but the SAT has fewer questions - 140 compared to the 215 on the ACT. The SAT also focuses heavily on vocabulary, while the ACT hones in on grammar and punctuation.
Source: www.examville.com