A problem in using outdoor advertising is that of getting reliable data on the number of people who actually see an advertisement.
使用户外广告的一个问题是如何获得关于实际看到广告的人数的可靠数据。
Reliable data, though, are scarce.
然而可靠的数据还是很少。
Without reliable data, prediction has no foundation.
没有可靠的资料,预报就没有基础。
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