Some customers pay as much as 25%, more than six times the base rate.
The index is measured from a base rate of 100 when it was first recorded in 1996.
The base rate in Poland stands at 6.00% and was last raised a quarter percentage point in May.
Part of the explanation is that the "base rate" set by the central bank in Malawi is high.
Interest rates are quite low but so is inflation: in real terms the base rate is around 3%.
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And in any case the ultra-low central-bank base rate will eventually have to rise, pushing up borrowing costs.
But borrowers beware: a further rise in the base rate, taking it to 6.0%, still seems highly likely.
But if the base rate were to stay at 5.25%, inflation would be below the target in early 2010.
Despite the Bank of England base rate remaining at 0.5%, Bank of Ireland is almost doubling its tracker rate.
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The Bank kept its base rate at 5% this week, but it will be looking to cut before long.
Although the base rate of interest is 4.5%, most credit cards charge upwards of 40%, plus an annual fee.
Italy has just had to pay 597 basis points (ie, 5.97%) above base rate in order to sell its bonds.
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Indeed, if the base rate were held unchanged at 5.75%, inflation would fall to around 1.75% in two years' time.
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When this is taken into account, the debt-servicing ratio is close to its previous peak when the base rate stood at 15%.
Even if it does, the base rate will have to rise, sooner rather than later, from its current emergency low of 0.5%.
Initially the informal process of getting a Libor rate was established specifically as a solution for getting a base rate on syndicated loans.
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But the three-month interbank rate soared, reaching 6% by the end of March, and credit conditions have accordingly tightened despite the lower base rate.
Today she uses New York-based Merchant Processing Services, which charges her a base rate of 1.61% on each bill, plus 20 cents per card swipe.
For the moment the Bank of England has plenty of scope to help the economy by cutting the base rate, which currently stands at 4.5%.
Last month, Bank of Ireland was criticised for increasing the interest rate on its Base Rate Tracker mortgages, despite the official rate remaining at 0.5%.
The more the economy weakens, the swifter the Bank of England is likely to be in lowering the base rate from its present level of 5.0%.
And that is when the bank is likely to start restoring more normal monetary settings, by raising the base rate from its all-time low of 0.5%.
Figures released this week for October when the bank had cut the base rate from 5% to 4.5% showed that people seemed to be doing better than firms.
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Over the weekend, it also increased the base rate for calculating import taxes on the precious metal, another measure to make imports costlier and bring down demand.
It is almost three years to the day since Bank slashed the official base rate - by 1.5 percentage points - in response to the banking crisis.
Suresh Hundia, president emeritus of the Bombay Bullion Association, said that the change in base rate would only have a marginal impact on local prices of the gold.
When the Bank of England dropped its base rate to 0.5%, it took the wind out of the sails of anyone looking for a decent rate of interest.
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Most analysts are forecasting, on balance, a quarter-point rise to 6.25% on Thursday, although base rate is not expected to rise beyond 6.5% before beginning to fall back.
Over the past two years, Polish central bankers have been hauling up the base rate for borrowing in order to keep up with the rapid pace of their economy's expansion.
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