Everyone feels sad from time to time. But what happens when that sadness lasts a long time? Is it just a passing feeling or the medical condition known as depression?

Many people do not know whether what they are feeling is a passing phase or a serious condition requiring treatment. They may not know where to turn to find out if they should get help.

Google is trying to change that by helping people recognize whether what they are feeling is actually depression. The company has teamed up with the National Alliance for Mental Illness, or NAMI, to offer a questionnaire to people who search for the word "depression" on Google's website.

The alliance says, "Clinical depression is a very common condition. In fact, approximately one in five Americans experience an episode in their lifetime." However, the alliance says, only about 50 percent of people who suffer from depression actually receive treatment."

Google has set up a short examination that can help people recognize whether they are suffering from clinical depression. The test will be offered to people who search for "depression" or "clinical depression" using Google on a mobile device.

The search results will show information on depression with a link to a series of questions, to help identify if someone has signs of depression.

The questionnaire is called Patient Health Questionnaire-9, or PHQ-9. Mental health experts use many of the same questions to help identify depression in their patients. While only a trained expert can identify depression, PHQ-9 can be a starting point to help you know whether to seek treatment.

NAMI explains, "We hope that by making this information available on Google, more people will become aware of depression and seek treatment to recover and improve their quality of life."

Using the depression questionnaire on Google

To take the test, search for "depression" in Google's search engine. The results should show an explanation about clinical depression. Below the explanation is a message that reads, "-> Check if you're clinically depressed."

Click that message to begin taking the questionnaire. Google says the test takes about five minutes to complete.

Touch the "Get Started" button to begin. At the end you will see a list of numbers from 0 to 27. It will show how likely you are to have signs of clinical depression.

The website also shows what to do next, based on the results of the questionnaire.

What else you need to know

PHQ-9 was developed for adults, not children. The questionnaire is available on Google search results only on phones and tablets, not other computers. If you search using a computer, you will see information about depression, but not the link to PHQ-9.

If you want to search for "depression" with a computer, or do not see the link to the questionnaire on your phone or tablet, you can search for "PHQ-9" on Google instead.

By taking this questionnaire on Google, you will be sharing information about yourself with the company, if you are in your Google account.

Google says that your answers will be kept private. They will not be shared with others. Google's privacy statement appears before the questionnaire:

Other ways to assess depression

There are other ways to help identify whether you are suffering from depression.

The PHQ-9 quiz is also available at the patientinfo.com website. You can take the test and see the results to help judge whether you suffer from depression.

Mental Health America, or MHA also offers an online depression test. After you take the test, MHA asks for personal information, but you can ignore that part by clicking on the word "Submit" without answering any of the questions. You will see the results after you take the test, with suggestions about what to do next.

The free Depression Test app for Android by Japps Medical also uses the PHQ-9 questionnaire. The app also gives users test results based on their symptoms of clinical depression. As with all computer software, be sure to read the permissions section before getting this app.

Also, the magazine Psychology Today has a depression test you can take online. Its test lasts about 20 minutes and gives you a short report. If you want a full report, you will have to pay about $5.

I'm Caty Weaver.