In the United States, at the end of 2018, there were 1,988 ETFs in the United States with combined assets of $3.3 trillion, accounting for 16% of the U.S. industry. Unit investment trusts (UITs) are issued to the public only once when they are created. UITs generally have a limited life span, established at creation. Investors can redeem shares directly with the fund at any time (similar to an open-end fund) or wait to redeem them upon the trust's termination. Less commonly, they can sell their shares in the open market. Unlike other types of mutual funds, unit investment trusts do not have a professional investment manager. Their portfolio of securities is established at the creation of the UIT. In the United States, at the end of 2018, there were 4,917 UITs with combined assets of less than $0.1 trillion. Some collective investment vehicles have the power to borrow money to make further investments; a process known as gearing or leverage. If markets are growing rapidly this can allow the vehicle to take advantage of the growth to a greater extent than if only the subscribed contributions were invested. However this premise only works if the cost of the borrowing is less than the increased growth achieved. If the borrowing costs are more than the growth achieved a net loss is achieved. This can greatly increase the investment risk of the fund by increased volatility and exposure to increased capital risk. Gearing was a major contributory factor in the collapse of the split capital investment trust debacle in the UK in 2002. Collective investment vehicles vary in availability depending on their intended investor base:  Some vehicles are designed to have a limited term with enforced redemption of shares or units on a specified date. Many collective investment vehicles split the fund into multiple classes of shares or units. The underlying assets of each class are effectively pooled for the purposes of investment management, but classes typically differ in the fees and expenses paid out of the fund's assets. These differences are supposed to reflect different costs involved in servicing investors in various classes; for example:  In some cases, by aggregating regular investments by many individuals, a retirement plan (such as a 401(k) plan) may qualify to purchase "institutional" shares (and gain the benefit of their typically lower expense ratios) even though no members of the plan would qualify individually. These also include Unit Trusts. One of the main advantages of collective investment is the reduction in investment risk (capital risk) by diversification. An investment in a single equity may do well, but it may collapse for investment or other reasons (e.g., Marconi).