Meaning of "mitentscheiden bei etw" (dat)?

Question

What does mitentscheiden bei etwas (dat) mean?

Specifically, I am not sure about the preposition. Does that mean this:
to decide something with someone about something (bei)

And is it then correct to say:

Wir haben bei dem Thema mitentschieden und das entschied.

Could you help me out with some appropriate examples in which to use this verb?

Answer 1

• entscheiden: to make a decision
• mitentscheiden: to have a say; being part of a group making a decision
• entscheiden mit jemandem: to make a decision with someone

None of this variations so far include the information about what you make a decision. There are basically two possibilities to add this bit. With the preposition bei or with an subordinate clause:

• (mit)entscheiden bei etwas
• (mit)entscheiden + [subordinate clause] (dass, ob, wann, ...)
• (mit)entscheiden + [relative clause] (wer, welcher)

Examples:

Die Zuschauer entscheiden, wer gewinnt. (The viewers decide who wins.)
Die Zuschauer entscheiden mit, wer gewinnt. (The viewers have a say in who wins.)
Die Zuschauer entscheiden mit der Jury, wer gewinnt. (The viewers and the jurors decide who wins.)

Die Juroren haben entschieden, dass der Angeklagte frei zu sprechen ist. (The jurors voted for an aquittal of the defendent.)

Patienten müssen sich entscheiden, ob sie operiert werden. (Patients have to make a decision whether or not they undergo a surgery.)
Patienten sollen mitentscheiden, ob sie operiert werden. (Patients should have a say in whether they undergo a surgery.)
Patienten sollen entscheiden können, wann sie operiert werden. (Patients should be able to decide when they undergo a surgery.)

Answer 2

You are correct with the translation. It means to decide something together with someone else. Entscheiden means to decide. The prefix mit means, that you decide something together with someone else.

You could say:

Wir haben bei dem Thema mitentschieden.

This would mean, that you decided something together with other people.

If you also want to say, what you have decided (for example you decided, that you want to buy some apples):

Wir haben mitentschieden, dass wir Äpfel kaufen.

You could also say, with which persons you have decided something.

Wir haben mit Person1, Person2 und Person3 entschieden, dass wir Äpfel kaufen.

In This case mitentscheiden gets sliced into mit and entscheiden.

Answer 3

Without mit:

Tom und ich haben bei der Wahl des Verkehrsmittels entschieden den Bus zu nehmen.

This is in English:

Tom and I decided in the choice of means of transport to take the bus.

So it was just Tom and me, but nobody else who met the decision.

With mit:

Tom und ich haben bei der Wahl des Verkehrsmittels mitentschieden.

English:

Tom and I participated in the decision in the choice of means of transport.

So it was not just Tom and me alone who decided if we would take the bus or the train. It was more than the two of us who met the decision together, and Tom and me were members of this deciding group.

So you could think of mitentscheiden as co-decide.