What is the most effective way to clearly identify required fields?

Required fields are essential for providing accurate and complete information. Yet, marking them can be tedious and time-consuming. Let's explore the pros and cons of marking required fields.

Overview

Assuming a form only ask for information it requires, it would make better sense to highlight behavior that go against that idea; which are optional fields. Avoiding the asterisk can also remove the ambiguity of asterisks.

"Literally including the phrase “optional” after a label is much clearer than any visual symbol you could use to mean the same thing. Someone may always wonder 'what does this asterisk mean?' and have to go hunting for a legend that explains things.” (Luke Wroblewski)

”While visual users usually see both the asterisk and the explanation at a glance and can connect them with each other intuitively, screen reader users have to manually search for the asterisk's purpose.”

“The implications for industry are quite profound. The single most important
message is to mandate fewer fields. More mandatory fields mean less voluntary
data disclosure whatever the sensitivity of a data item. Optional fields yield a
good data return. A company that wants to extract many details should use
optional fields but not highlight them. When users are unsure whether a field is
optional or mandatory, users would rather fill in a field than skip it.”
(Preibush, 2012)

Evidence and further readings

Here you can find a list of references to dive deeper on the subject.

• Luke Wroblewski
• Preibusch, S., Krol, K., & Beresford, A.R. (2012). The Privacy Economics of Voluntary Over-disclosure in Web Forms. 
• Adam Silver, 2018
• Accessibility Developer Guide, 2018