two group of people vs two groups of people

Some examples from the web:

• Two groups of people were changed.
• You can do an experiment where you give two groups of people a hundred dollars in the morning.
• Two groups of people were recently arrested in Northern Cyprus under Article 171 of the 1929 Criminal Code, pursuant to which male homosexuality is a criminal offence.
• Using a detailed questionnaire, it interviewed two groups of people - members of the public and professional cash handlers - who examined all ten design series. Altogether, some 2,000 people across Europe were surveyed.
• You take two groups of people who are similar in all relevant respects, and you remind one group that they're going to die but not the other, then you compare their behavior.
• There were two groups of people who were interested in this: planetary scientists, who want to look up, and then other people, who wanted to look down.
• It was starting to feel like the world was divided into two groups of people... those on their phones and those not on their phones.
• Interestingly, there are data on these two groups of people, data on how happy they are.
• Or is it wrong to help two groups of people at once?
• You guys, when two groups of people don't understand each other, you can't just try and diffuse
• So here's an example: One recent study took two groups of agnostics, that is people who are undecided in their religious beliefs.
• There's two groups of buildings.
• There were two groups of boys that needed saving.
• There are two groups of what we may term non-official languages.
• So Cathy took two groups of mice.
• Replies were received from the two groups of Union producers which are the applicants.
• Corbin believed that there were two groups of witches in Sleepy Hollow...
• two groups of teenagers inside a fast-food restaurant...
• The programme covers two groups of beneficiaries:
• And he gave the case out, both of them, to two groups of students.

Some examples from the web:

• You can do an experiment where you give two groups of people a hundred dollars in the morning.
• It was starting to feel like the world was divided into two groups of people... those on their phones and those not on their phones.
• There were two groups of people who were interested in this: planetary scientists, who want to look up, and then other people, who wanted to look down.
• You take two groups of people who are similar in all relevant respects, and you remind one group that they're going to die but not the other, then you compare their behavior.
• Interestingly, there are data on these two groups of people, data on how happy they are.
• Or is it wrong to help two groups of people at once?
• You guys, when two groups of people don't understand each other, you can't just try and diffuse
• Two groups of people were recently arrested in Northern Cyprus under Article 171 of the 1929 Criminal Code, pursuant to which male homosexuality is a criminal offence.
• Two groups of people were changed.
• Using a detailed questionnaire, it interviewed two groups of people - members of the public and professional cash handlers - who examined all ten design series. Altogether, some 2,000 people across Europe were surveyed.