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No. of Recommendations: 5
Moody's publishes charts of cumulative, average, 5-yr, default-rates that look (roughly) like this:
Aaa     1
Aa      2
A       3
Baa     4
Ba      8
B      20
Caa-c  33

There's enough similarities to a Fib series that the rates could be "normalized" as follows:
Aaa     1
Aa      2
A       3
Baa     5
Ba      8
B      21
Caa-c  34

Frankly, when things are done that way, I don't believe the huge jump between 'Ba' and 'B, nor do I believe that 'Baa' fares as well as they report. 
So let's make some adjustments:
Aaa     2
Aa      3
A       5
Baa     8
Ba     13
B      21
Caa-c  34

There are so few triple Aaa's these days that it doesn't matter whether the default rate is reported accurately. But the rest of the schedule looks good to my eye. 
Now, let's make one further adjustment and notch the whole schedule down by one step:
Aaa      3
Aa       5
A        8
Baa     13
Ba      21
B       34
Caa-C   55

What the above schedule says to me is that all bonds have some risk, but anything lower than single B is more likely than not to be losing a trade.
Yeah, the yields offered by the triple Caa's (and lower) can be tempting. But with that tranche, you're buying lottery tickets, not "investing" *unless* 
you really, really are a superior credit-analyst who specializes in "deep value" situations. 

Said another way, bottom-tier triple BBB's are junk pretending to be invest-grade and --typically-- would lose that status if the rating agencies didn't fear the consequences 
of being honest. Double BB's, OTOH, generally offer a lot of value. Triple CCC's (and lower) are the "pennies stocks" of the bond world and should be avoided except by specialists.

Caveats and exceptions: 
As we all know, and as the book/movie, The Big Short, made clear, when it's in their interests to do so, the rating agencies will assign a top-tier rating to trash. 
And other times, even when they are doing an honest, impartial job of assigning a rating, the rating might not be timely or correct. 

If you want 'timely", then you need to depend on how traders are rating the debt through their pricing. (I.e., the "market-implied" rating.) 
And that, too, might not be correct, either.

Also --and crucially--  cumulative, average, 5-year default rates are only meaningful when used to assess the likelihood of a basket of similarly rated bonds failing, 
not individual credits. But if you're buying baskets of bonds, i.e., carrying hundreds of positions, not just the dozen or so most FI limit themselves to, 
then "average expectations" can become useful risk-assessment tool.

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