A great example of what Sean and I would define as combination music is Nelly and Tim McGraw's crossover hit "Over and Over." This was also a prime song to look at because its reviews vary, expressing divergent opinions on the country/rap song.
Note: Snapshots of reviews will appear small because of formatting issues, but click on them and they will become large enough to read.
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Here somnambulist reveals one effect that combination music can have; it can open listeners of one genre up to a new one that they may embrace. This reviewer even uses the word "discovered." A takeaway from this review is that one need not necessarily be a fan of all genres that are combined in combination music before hearing it to enjoy it.
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In this review, Mikey the Great aptly points out that for an artist, the risk of dabbling in a new genre is not a fleeting one that fades away quickly. It can impact their image long past the time that a single is released. Mikey the Great doesn't explicitly say "image" here, but his mention of street cred can be used as a substitute. Another key point to denote here is that Mikey the Great was a fan of Nelly before this song came out. Certainly, there will always be listeners who write terrible reviews for artists they already disliked, but the true fans whose opinions change for the worse are really the ones with whom artists must be concerned when attempting to cross over genre lines.
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In comparing country and rap to peanutbutter and pickles, moonshine19's review shows that not all the animosity tied into disliking a form of combination music is placed on the shoulders of the artist or group.
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Here is another example of a reviewer who mentioned image.
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While the examples I provided here were mostly negative, eight out of 11 references to the song in the reviews were positive. There is clearly response bias involved (chances are those who didn't buy it/didn't like it were less likely to respond), but to be fair I included the breakdown.
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To touch on the mashup side of combination music, I looked at reviews of "Frontin' on Debra (DJ Reset Mash-Up)," a combination of Beck's "Debra" and Pharrell's "Frontin.'"

This one struck me because in it, juleshills actually refers to the song as a mashup (rare in these reviews) and outlines what a mashup should do.
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This review shows a major distinction between crossover songs and mashups. When it comes to mashups, artists are not held accountable because their music is fused with another's music often without consent.