Dear Economist,
为何我在繁华商业街上找一条适合的裤子就那样难呢？作为一个腰节33-34英寸、高5英尺11英寸的男士，我觉得我们的体型算是中等。然而，适合的衣服好像一直脱销。经济学理论能讲解这一点吗？
Why is it so hard for me to find a properly fitting pair of trousers on the high street? As a 33-34in waisted, 5ft 11in male, I consider myself of average build. Yet appropriate attire1 always seems to be out of stock. Can economic theory shed any light on this?
焦虑的买裤人
Concerned Trouser Consumer
亲爱的买裤人：
Dear Trouser Consumer,
这里显然可以运用的经济剖析是，哈罗德o霍特林在1929年发表的关于商品差异化的著作。霍特林设想有两个冰淇淋小贩，他们试图确定摊位在海滩上的地方。
The obvious economic analysis here is Harold Hotelling's 1929 classic on product differentiation2. Hotelling imagined two ice-cream stands trying to decide where to locate on a beach.
霍特林的结论是，他们的摊位最后将背靠背地设置在（海滩）中央，每一个买家都会去近期的摊位。类似，主要政治党派也倾向聚集在政治谱系的中央。
His conclusion was that they would end up side-by-side in the middle, with each customer going to the nearest stall. Similarly, major political parties tend to cluster in the centre of the political spectrum3.
大家不会看到有公司专门生产单一尺码的裤子；假如发生这样的情况，你就会有很多合适我们的裤子。一个较好的讲解是，公司进行着策略性商品差异化，以防止角逐。对于霍特林设想的冰淇淋小贩而言，这意味着将摊位设在海滩的两端。对于角逐的公交公司而言，这意味着一家公司在整点和半点发车，另一家在整点前后一刻钟发车。
We do not see companies specialising in a single size of trouser; if we did, you would be swamped with pairs that fitted you well. A better explanation may be that companies are engaging in strategic product differentiation to avoid competition. For Hotelling's ice-cream vendors4, that would mean locating at opposite ends of the beach. For rival bus companies it would mean one service departing on the hour and half hour, the other at quarter to and quarter past.
在每一个案例中，目的都是赋予两家公司提价的能力，同时又不让它们失去太多的买家。
In each case the aim is to give both companies the ability to raise prices without losing too many customers.
在你的案例中，策略性商品差异化意味着一家服饰店卖腰节32英寸、36英寸和40英寸的裤子。另一家店卖腰节30英寸、34英寸和38英寸的裤子。
In your case, strategic product differentiation would mean that one clothes store offers trousers with a 32in, 36in and 40in waist. The other offers 30in, 34in and 38in.
假如你去了错误的店，你就从来不会得到认可的结果；正确的店会提供适合的服饰，但因为缺少角逐，价格会偏贵。我承认这是一种古怪的经济理论。
If you are shopping at the wrong store you will never be satisfied; the right store would supply clothes to fit, but prices would be higher for lack of competition. It is a bizarre economic theory, I agree.
但这能讲解你常常受挫是什么原因。
But it would explain your commonplace frustration5.