If you are building a properly diversified stock portfolio, the chances are some of your picks will perform badly. Long term Prothena Corporation plc (NASDAQ:PRTA) shareholders know that all too well, since the share price is down considerably over three years. Sadly for them, the share price is down 61% in that time. And over the last year the share price fell 46%, so we doubt many shareholders are delighted. The falls have accelerated recently, with the share price down 20% in the last three months.

If the past week is anything to go by, investor sentiment for Prothena isn't positive, so let's see if there's a mismatch between fundamentals and the share price.

See our latest analysis for Prothena

Prothena wasn't profitable in the last twelve months, it is unlikely we'll see a strong correlation between its share price and its earnings per share (EPS). Arguably revenue is our next best option. When a company doesn't make profits, we'd generally hope to see good revenue growth. Some companies are willing to postpone profitability to grow revenue faster, but in that case one would hope for good top-line growth to make up for the lack of earnings.

Over the last three years, Prothena's revenue dropped 12% per year. That's not what investors generally want to see. The share price decline of 17% compound, over three years, is understandable given the company doesn't have profits to boast of, and revenue is moving in the wrong direction. Of course, it's the future that will determine whether today's price is a good one. We'd be pretty wary of this one until it makes a profit, because we don't specialize in finding turnaround situations.

The image below shows how earnings and revenue have tracked over time (if you click on the image you can see greater detail).

You can see how its balance sheet has strengthened (or weakened) over time in this free interactive graphic.

A Different Perspective

Investors in Prothena had a tough year, with a total loss of 46%, against a market gain of about 24%. Even the share prices of good stocks drop sometimes, but we want to see improvements in the fundamental metrics of a business, before getting too interested. Longer term investors wouldn't be so upset, since they would have made 0.8%, each year, over five years. It could be that the recent sell-off is an opportunity, so it may be worth checking the fundamental data for signs of a long term growth trend. It's always interesting to track share price performance over the longer term. But to understand Prothena better, we need to consider many other factors. Consider for instance, the ever-present spectre of investment risk. We've identified 2 warning signs with Prothena , and understanding them should be part of your investment process.

For those who like to find winning investments this free list of undervalued companies with recent insider purchasing, could be just the ticket.

Please note, the market returns quoted in this article reflect the market weighted average returns of stocks that currently trade on American exchanges.

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This article by Simply Wall St is general in nature. We provide commentary based on historical data and analyst forecasts only using an unbiased methodology and our articles are not intended to be financial advice. It does not constitute a recommendation to buy or sell any stock, and does not take account of your objectives, or your financial situation. We aim to bring you long-term focused analysis driven by fundamental data. Note that our analysis may not factor in the latest price-sensitive company announcements or qualitative material. Simply Wall St has no position in any stocks mentioned.