Which of the following is an example of qualitative data in Six Sigma?

Qualitative data refers to non-numeric information that describes qualities or characteristics. Customer feedback is a prime example of qualitative data in Six Sigma as it includes opinions, feelings, and perceptions about a product or service. This type of data can provide insights into customer satisfaction, preferences, and areas for improvement, which are crucial for understanding the broader context of project goals and outcomes.

In contrast, the other options represent quantitative data, which is numerical and can be measured. Monthly sales figures provide specific amounts related to sales, production times offer measurable durations, and machine operating temperatures give exact measurements relevant to performance. While these quantitative metrics are vital for analysis and process control in Six Sigma, they do not convey the qualitative aspects that customer feedback does, thereby making customer feedback the correct example of qualitative data.