My first comment will be that it is better to add one reference too many than one too few. Reviewers or editors might suggest to remove references during the review process if they seem superfluous. 

In some cases, it might be more useful to add references to quite basic concepts. One such instance is when papers are directed at least partially to people outside the field. In such cases it may be sufficient to point at a basic book in the field, as an example to point to a good book on statistics regarding the Poisson distribution (the book has to provide some good information on the topic of course). This can help people who may have a different background to get deeper into the topic. 

In general there is of course a fine line where references are or are not needed and with time one learns to identify this line better. What you can do is to simply look at other papers that you have read and see where they place the line and try to follow their example. That will likely form a very good basis for finding the right level of referencing. But, be aware that the line is different depending on the audience of the publication where the paper is published and you should briefly check to see what is applicable in each journal where you publish.

Citation to well-known results and reference to user manuals
[Drupal 7] 

I have a block I created in Views called "handy_tips-block_1".

Now I want to insert it into the body field of my node.

How do I do this?

I found a guide on here, which suggests to use the following code:

&lt;?php
print $block = module_invoke('views', 'block_view', 'handy_tips-block_1');
print render($block['content']['#content']);
?&gt;


However, all what is printed out are the words "Array".
(I have enabled the PHP input filter for the body field).

Note: in the posted code, I have also tried substituting 'views' with various items such as "block". I have used 'handy_tips-block_1' as the ID, because that is what is displayed on the configuration link of the block administration page. 
