I'm the only person who manages Nagios Core & relevant services. Once deployed, I provided training to some Linux engineers so that they can manage it during my unavailability.  

So, everything is in-house. Also, we have a roll back option to restore the last configuration if something goes wrong. In most cases, junior Linux engineers make a mistake in Nagios host/service configuration files. So, prior to making any changes, they need to make a backup copy under the same directory level.

Deployment was done in-house.

We implemented the solution in-house. 

I pretty much followed my own notes from previous experience. So I followed a document and deployed the solution. One person is enough for the deployment and maintenance of the solution. 

I completed the installation myself.

We primarily implement through vendor teams.

If a person has basic or in-depth level knowledge of all required network/server equipment, than he/she can easily implement Nagios.

Also, it is advisable to have proper knowledge of SNMP v1/2/3 with Nagios agents for Windows and Unix-like OS.

The implementation was completed in-house because previously it was an open source tool. We can configure it to our own needs, based on our own requirements. The only task we had to complete is creating the script and using the tool. We had alerts created for our mobile phones, email and dashboard.

We maintain Nagios Core in-house.

It is an open source tool which provides capability to customize it according to the needs. So internally you need to have expertise to consume its servers unless you go for the paid options.

We deployed it ourselves. Its maintenance is done by one or two people.

We did an in-house installation.

You do not need a vendor. I have always deployed it myself.

I implemented it by myself.

This was all completed in-house.

We didn't have installers or another company assist us. We handled the implementation ourselves.

We did it in-house. There is no extra effort needed if you just go through the regular installation instructions.

It was all done by us. We were given time to do our own research and through regular testing, trials and errors, we finally implemented it. My advice is to not be scared by the need to configure everything through the CLI. It's actually quite fun and rewarding when you see your monitoring system finally up and you know you can count on it to give you a heads up on alerts before something nasty happens to your server.

It's very easy to install Nagios as its package is provided by many Linux distributions and there are plenty of documentation online.

I implemented it all by myself. 

I did it myself.

We used an in-house team. I would advise you to learn from users past experience as it always helps.

I did it myself so no vendor is involved

Implementation requires knowledge on the production.

We did it in-house.