Online surveys
Online surveys can be a quick, cost-free and easy way to gather user requirements on a large scale. Data collected can be both qualitative and quantitative and should include both structured (e.g. multichoice) and unstructured (free text) questions. Questions should be well considered to avoid redundant questions and keep to a maximum of 10 questions. Sample sizes will depend on the scale of the population. The minimum sample size should be 100 participants with a 10% margin of error. This would give you the optimum confidence level. Sample sizes should be no larger than 1000.
Surveys can be a great way to gather additional qualitative insights by using open-ended questions. See Neilson Normans tips for Qualitative Surveys
A/B Testing
A/B Testing encourages experimentation, facilitates improvement, validates assumptions and produces data-driven design. It is used as a method to test prototypes of user journeys and is usually unmoderated but researchers may help facilitate this process.