Criteria in which we can select whether we should go for manual or automation testing

1. If we have to only testing once: We should Use manual testing –if only one function of one mobile app is being tested, it doesn’t make sense to spend the time and energy to create an automated testing script. Although manual testing may take longer than running an automated test, script creation takes time. If there isn’t a case for re-use, there is really no need to tie up automation experts with script creation for a single test case.
2. Regression tests: We should Use automation testing – Each time a developer releases new code, a regression test is needed. Because regression tests are repeated, automated testing is a perfect fit. While there is some time needed on the front end to develop test scripts, testers will save time because they won’t have to start from scratch with the test each time. We should try to create our script in such a way that most common functionality test scripts should be created in such a way that we can reuse them in other projects also whenever we encounter same problem.
3. Complex tests scenarios: We should Use automation testing – Complex tests have multiple components that need to be tested at once. After testers familiarize themselves with the app’s functionality, an automated test script can help them run through a variety of tests quickly to speed the testing process. 
4. New functionality: We should Use manual testing – because If an app contains new functionality, it should be tested manually. With new functionality, testers won’t know what type of automated test script to write prior to performing a manual test.