Final answer:
The common effectiveness rates for spermicides used alone are likely higher than an 18-21 percent failure rate, which is the rate for condoms with typical use. Spermicides are less effective when not combined with barrier methods.
Step-by-step explanation:
The closest range of common effectiveness rates for spermicides used alone is not directly listed, but we can infer that it lies closer to methods with higher failure rates since spermicides are less effective when not used in conjunction with barrier methods. Given the context about the typical use of contraception methods, spermicides would likely have a typical effectiveness rate lower than condoms, which have an 18 percent failure rate for condoms and a 21 percent failure rate for vagina condoms with typical use. Thus, the range for spermicides alone might be reasonably estimated to fall around or just beneath the 21 percent mark, making it significantly less effective than other forms of contraception like birth control pills, IUDs, or condoms.