Sanity Testing

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A sanity test or sanity check is a basic test to quickly evaluate whether a claim or the result of a calculation can possibly be true. The point of a sanity test is to rule out certain classes of obviously false results, not to catch every possible error. In arithmetic, for example, when multiplying by 9, using the divisibility rule for 9 to verify that the sum of digits of the result is divisible by 9 is a sanity test – it will not catch every multiplication error; however it’s a quick and simple method to discover many possible errors.

In computer science, a sanity test is a very brief run-through of the functionality of a computer program, system, calculation, or other analysis, to assure that the system or methodology works as expected, often prior to a more exhaustive round of testing.

Mathematical

A sanity test can refer to various order-of-magnitude and other simple rule-of-thumb devices applied to cross-check mathematical calculations. For example:

• If one were to attempt to square 738 and calculated 53,874, a quick sanity check could show that this result cannot be true. Consider that 700 < 738, yet 7002 = 721002 = 490000 > 53874. Since squaring positive numbers preserves their inequality, the result cannot be true, and so the calculation was bad. The correct answer, 7382 = 544,644, is more than 10 times higher than 53,874, and so the result had been off by an order of magnitude.
• In multiplication, 918 × 155 is not 142135 since 918 is divisible by three but 142135 is not (digits add up to 16, not a multiple of three). Also, the product must end in the same digit as the product of end-digits 8×5=40, but 142135 does not end in “0” like “40”, while the correct answer does: 918×155=142290. An even quicker check is that the product of even and odd numbers is even, whereas 142135 is odd.
• When talking about quantities in physics, the power output of a car cannot be 700 kJ since that is a unit of energy, not power (energy per unit time).

Software development

In software development, the sanity test (a form of software testing which offers “quick, broad, and shallow testing”) determines whether it is reasonable to proceed with further testing.

Software sanity tests are commonly conflated with smoke tests. A smoke test determines whether it is possible to continue testing, as opposed to whether it is reasonable. A software smoke test determines whether the program launches and whether its interfaces are accessible and responsive (for example, the responsiveness of a web page or an input button). If the smoke test fails, it is impossible to conduct a sanity test. In contrast, the ideal sanity test exercises the smallest subset of application functions needed to determine whether the application logic is generally functional and correct (for example, an interest rate calculation for a financial application). If the sanity test fails, it is not reasonable to attempt more rigorous testing. Both sanity tests and smoke tests are ways to avoid wasting time and effort by quickly determining whether an application is too flawed to merit any rigorous testing. Many companies run sanity tests and unit tests on an automated build as part of their development process.

The Hello world program is often used as a sanity test for a development environment. If Hello World fails to compile or execute, the supporting environment likely has a configuration problem. If it works, the problem being diagnosed likely lies in the real application being diagnosed.

Another, possibly more common usage of ‘sanity test’ is to denote checks which are performed within program code, usually on arguments to functions or returns therefrom, to see if the answers can be assumed to be correct. The more complicated the routine, the more important that its response be checked. The trivial case is checking to see that a file opened, written to, or closed, did not fail on these activities – which is a sanity check often ignored by programmers. But more complex items can also be sanity-checked for various reasons.

Examples of this include bank account management systems which check that withdrawals are sane in not requesting more than the account contains, and that deposits or purchases are sane in fitting in with patterns established by historical data – large deposits may be more closely scrutinized for accuracy, large purchase transactions may be double-checked with a card holder for validity against fraud, ATM withdrawals in foreign locations never before visited by the card holder might be cleared up with him, etc.; these are “runtime” sanity checks, as opposed to the “development” sanity checks mentioned above.

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