The last year has been a rather exciting one for investors in advertising and PR specialist Next Fifteen (LSE: NFC). That’s because the company’s shares have risen by a whopping 37% as the outlook for the global economy has improved. Furthermore, the company has been able to increase its earnings by 78% and 36%, respectively, in the last two years, which has clearly caused investor sentiment towards Next Fifteen to improve dramatically.

Looking ahead, the uncertain outlook for the global economy could cause the pace of Next Fifteen’s share price rise to moderate somewhat. US interest rate increases appear very likely over the next year and this could cause investor sentiment towards cyclical stocks such as Next Fifteen to weaken. However, with the company having a sound business model that’s well-diversified, it may be able to continue to deliver upbeat growth numbers in the long run.

Certainly, earnings growth forecasts of 9% this year and 10% next year have huge appeal. And with Next Fifteen trading on a price-to-earnings growth (PEG) ratio of just 1.3, now seems to be an excellent time to buy a slice of it for the long run.

Worth a look

While Next Fifteen has enjoyed a strong year of share price growth, shares in healthcare company Clinigen (LSE: CLIN) have disappointed. That’s because they’ve fallen by 15% during the period despite Clinigen recording three successive years of double-digit growth.

In fact, Clinigen’s earnings per share have risen from 13.4p in 2012 to 28p in 2015. That’s an annualised rise of almost 28% and shows that Clinigen remains a very strong growth play. And with the company forecast to increase its bottom line by 21% in each of the next two years, investor sentiment could begin to pick up over the coming months.

That’s especially the case since Clinigen trades on a PEG ratio of just 0.6, which indicates that the market hasn’t yet begun to price-in its improving financial outlook. And with Clinigen having a beta of just 0.7, its shares could offer a less volatile shareholder experience in the short run. With the potential for increased uncertainty in the coming months, this could prove to be a major ally for the company’s investors.

Waiting game

Meanwhile, Sirius Minerals (LSE: SXX) has recorded a share price rise of 28% since the turn of the year, which brings its five-year capital gain to 100%. Clearly, that’s an impressive return, but Sirius Minerals has been a relatively risky investment during that time, with its success being heavily reliant on the approval of a major potash mine in York.

Although approval for the mine has now been granted, Sirius Minerals remains a relatively high-risk stock to own. It requires vast financing for such a large project and while investor sentiment towards the resources sector has improved of late, the commodity price collapse of recent years could still make fundraising more difficult for the firm.

Due to this, Sirius Minerals may be a stock to watch rather than buy at the moment – especially with a number of other smaller companies offering high growth and low valuations.

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Peter Stephens owns shares of Clinigen and Next Fifteen Communications. The Motley Fool UK has recommended Clinigen. We Fools don't all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors.