What Word, or Which Word, Should You Use?

In conversational language and informal writing, the pronouns what and which are often used interchangeably as determiners—words used in asking about or referring to people, places, or things. However, the careful writer will distinguish between the general determiner what and the specific determiner which. Note the subtle difference in the following pairs of examples:

1.The change management plan should identify what skills the transition requires.The change management plan should identify which skills the transition requires.

2.The plan should define what value drivers will be measured.The plan should define which value drivers will be measured.

3.This should be followed by a gap assessment to determine what staffing and external support adjustments are required.This should be followed by a gap assessment to determine which staffing and external support adjustments are required.

4.Regardless of what method you choose, you’ll be helping conserve water.Regardless of which method you choose, you’ll be helping conserve water.

5.These pacts lay out what types, level, and duration of support an organization will continue to provide.These pacts lay out which types, level, and duration of support an organization will continue to provide.

6.What facets of the model should be implemented first due to their importance?Which facets of the model should be implemented first due to their importance?

7.What TV programs are your favorites?Which TV programs are your favorites?

Notice that in the first sentence in each pair, only what is correct if the noun or noun phrase that follows it is omitted and that the second sentence in each pair has a more decisive impact as a question or statement. In the following examples, you’ll see that when considering whether to replace what with which, which is appropriate only when further revision occurs that provides more specific information, as in the following examples:

8.She obviously does not know what we are referring to.She obviously does not know which case we are referring to.

9.The management plan should identify what various departments need to be familiar with.The management plan should identify which policies various departments need to be familiar with.

10.Will you help me decide what looks best on me?Will you help me decide which hat looks best on me?