Word strengths and testing out skills
I'm having some trouble. I know some French, and have known a specific amount of French for three years without forgetting any of it. When I decided to start Duolingo, it was because I wanted to build upon my French that I was learning in school. I started Duolingo, and I tested for French and tested out of the first nine skills. This worked really well, adn I thought I wouldn't have to worry about them. I started learning some more French, which I am excited about, but then I saw "Words" at the top. I clicked it, and I saw all my words that it said I "learned from Duolingo." Some of the words that I already knew were on there. I guess that can be excused. However, about a week later, I saw some words were not at full strength. Many of those words were the ones that I tested out of. To me, that is ridiculous. If I tested out of skills and Duolingo says that I already knew those words, then Duolingo should always have them at full strength. Also, some new words I'm learning are really bugging me because they are at strength one. This makes me want to put them back to full. That's OK. What I'm complaining about is that when I hit the strengthen button on the skill it is found in. Sometimes that word wont appear during the lesson at all. Also, sometimes, when it does appear, even though I practice it, the strength doesn't change. I used to have German on here. I had the word "pferd" (horse) at strength one. I practiced it 28 times before the strength went from one to full. It was ridiculous. Sometimes, Duolingo saying some words aren't at full strength can be wrong. It discourages me because Duolingo makes me feel like I am learning the language incorrectly.
I wouldn't look at the word list; I'd look at the tree. If a module you've completed isn't gold any more, you can go to the module and click on Strengthen rather than repeating every lesson. If you really know the words, it should be easy to just answer the seventeen questions to turn the module gold again.
The more often you hit a module, the slower the decay rate for the words in the module.
It ain't perfect, but it's probably more efficient than working one word at a time.
Like what @FredrikVC said, I'd keep your tree golden more so than your words list. However, the strength bars only indicate when Duolingo thinks you should be practicing the word again, not how well you know the word. The bars aren't meant to tell you that you don't know something, but rather to tell you that you need to practice this word again so it stays fresh in your mind.