How to automate the tracking of ratios and other values for a set of stocks or ETFs?
Spreadsheets need not be static, they can pull data from the web.  This article describes the method you seek.
Can they change the weights? Yes. Will they? It depends. are ETF's fixed from their inception to their de-listing? It's actually not possible for weights to be fixed, since different assets have different returns. So the weights are constantly changing as long as the market is moving. Usually after a certain period or a substantial market move, fund managers would rebalance and bring the weights back to a certain target. The target weights - what your question is really about - aren't necessarily the same as the initial weights, but often times they are. It depends on the objective of the ETF (which is stated in prospectus). In your example, if the manager drops the weight of the most volatile one, the returns of the ETF and the 5 stocks could be substantially different in the next period. This is not desirable when the ETFs objective is to track performance of those 5 stocks.  Most if not all ETFs are passively-managed. The managers don't get paid for active management. So they don't have incentive to adjust the weights if their funds are tracking the benchmarks just fine. "The property tax valuation and the fair market price are NOT one and the same.  They track each other, correlate to each other, but are almost NEVER the same number. In some parts of the USA, a municipality has to re-assess property tax values every ten years.  In these places, the tax value of a property is on something like a 10-year moving average, NOT on the volatile daily market price. EDIT:  It is easy to fall into the ""trap"" of thinking that property tax valuation is intended to represent fair market value.  It's INTENT is to provide an accurate (or, as accurate as possible) RELATIVE VALUATION of your property compared to the other properties in the municipality. The sum of all the property values is the tax base of the municipality.  When the town budget (which is paid in part via property taxes) is set, the town simply divides the tax base into the budget total to arrive at the ratio of tax-to-collect, to the tax base, also called the ""tax rate per thousand dollars of valuation.""  i.e. if the town tax base is US$10,000,000, and the town budget is US$500,000, then the ratio is 0.05, or $50 per thousand dollars of valuation.  If your property is assessed at US$100,000, then you would pay 100 x $50, or $5000 in property taxes that year. Since this is the goal of the property tax valuation, NOT deciding what your house is worth on the open market, then we are left with the question of ""why use the market value of a house for property assessment?"" and the answer is that of all the various schemes and algorithms you can try, ""fair market value"" is the easiest and most accurate...IF TIME FLUCUTATIONS ARE TAKEN OUT.  For example, if I buy a house in a development for $250,000 today, and next summer the housing market crashes, and you buy the identical house next door to me for $150,000, it does NOT stand to reason that you should pay less taxes than me, because your house is ""worth"" $100,000 less.  In fact, BOTH our houses are worth $100,000 less. What matters most in property tax valuation isn't the actual number, but rather, is YOUR valuation the same as other essentially similar properties in your tax base?  Getting the RELATIVE ratio of value between you and your neighbors correct is the goal of property tax valuation." "First, what structure does your index fund have?  If it is an open-end mutual fund, there are no bid/ask spread as the structure of this security is that it is priced once a day and transactions are done with that price.  If it is an exchange-traded fund, then the question becomes how well are authorized participants taking advantage of the spread to make the fund track the index well?  This is where you have to get into the Creation and Redemption unit construct of the exchange-traded fund where there are ""in-kind"" transactions done to either create new shares of the fund or redeem out shares of the fund.  In either case, you are making some serious assumptions about the structure of the fund that don't make sense given how these are built. Index funds have lower expense ratios and are thus cheaper than other mutual funds that may take on more costs.  If you want suggested reading on this, look at the investing books of John C. Bogle who studied some of this rather extensively, in addition to being one of the first to create an index fund that became known as ""Bogle's Folly,"" where a couple of key ones would be ""Common Sense on Mutual Funds: New Imperatives for the Intelligent Investor"" and ""Bogle on Mutual Funds: New Perspectives for the Intelligent Investor."" In the case of an open-end fund, there has to be a portion of the fund in cash to handle transaction costs of running the fund as there are management fees to come from running the fund in addition to dividends from the stocks that have to be carefully re-invested and other matters that make this quite easy to note.  Vanguard 500 Index Investor portfolio(VFINX) has .38% in cash as an example here where you could look at any open-end mutual fund's portfolio and notice that there may well be some in cash as part of how the fund is managed.  It’s the Execution, Stupid would be one of a few articles that looks at the idea of ""tracking error"" or how well does an index fund actually track the index where it can be noted that in some cases, there can be a little bit of active management in the fund. Just as a minor side note, when I lived in the US I did invest in index funds and found them to be a good investment.  I'd still recommend them though I'd argue that while some want to see these as really simple investments, there can be details that make them quite interesting to my mind. How is its price set then? The price is computed by taking the sum value of all the assets of the fund minus the liabilities and divided by the number of outstanding shares.  The price of the assets would include the closing price on the stock rather than a bid or ask, similar pricing for bonds held by the fund, derivatives and cash equivalents.  Similarly, the liabilities would be costs a fund has to pay that may not have been paid yet such as management fees, brokerage costs, etc. Is it a weighted average of all the underlying stock spreads, or does   it stand on its own and stems from the usual supply & demand laws ? There isn't any spread used in determining the ""Net Asset Value"" for the fund.  The fund prices are determined after the market is closed and so a closing price can be used for stocks. The liabilities could include the costs to run the fund as part of the accounting in the fund, that most items have to come down to either being an asset, something with a positive value, or a liability, something with a negative value. Something to consider also is the size of the fund.  With over $7,000,000,000 in assets, a .01% amount is still $700,000 which is quite a large amount in some ways." Now, if I'm not mistaken, tracking a value-weighted index is extremely easy - just buy the shares in the exact amount they are in the index and wait. Yes in theory. In practise this is difficult. Most funds that track S&P do it on sample basis. This is to maintain the fund size. Although I don't have / know the exact number ... if one wants to replicate the 500 stocks in the same %, one would need close to billion in fund size. As funds are not this large, there are various strategies adopted, including sampling of companies [i.e. don't buy all]; select a set of companies that mimic the S&P behaviour, etc. All these strategies result in tracking errors. There are algorithms to reduce this. The only time you would need to rebalance your holdings is when there is a change in the index, i.e. a company is dropped and a new one is added, right? So essentially rebalance is done to; If so, why do passive ETFs require frequent rebalancing and generally lose to their benchmark index? lets take an Index with just 3 companies, with below price. The total Market cap is 1000  The Minimum required to mimic this index is 200 or Multiples of 200. If so you are fine.    More Often, funds can't be this large. For example approx 100 funds track the S&P Index. Together they hold around 8-10% of Market Cap. Few large funds like Vangaurd, etc may hold around 2%. But most of the 100+ S&P funds hold something in 0.1 to 0.5 range. So lets say a fund only has 100. To maintain same proportion it has to buy shares in fraction. But it can only buy shares in whole numbers. This would then force the fund manager to allocate out of proportion, some may remain cash, etc. As you can see below illustrative, there is a tracking error. The fund is not truly able to mimic the index.  Now lets say after 1st April, the share price moved, now this would mean more tracking error if no action is taken [block 2] ... and less tracking error if one share of company B is sold and one share of company C is purchased.  Again the above is a very simplified view. Tracking error computation is involved mathematics.  Now that we have the basic concepts, more often funds tracking S&P; Thus they need to rebalance. About 10 years ago, I used to use MetaStock Trader which was a very sound tool, with a large number of indicators, but it has been a number of years since I have used it, so my comments on it will be out of date. At the time it relied upon me purchasing trading data myself, which is why I switched to Incredible Charts. I currently use Incredible Charts which I have done for a number of years, initially on the free adware service, now on the $10/year for EOD data access. There are quicker levels of data access, which might suit you, but I can't comment on these. It is web-based which is key for me. The data quality is very good and the number of inbuilt indicators is excellent. You can build search routines on the basis of specific indicators which is very effective. I'm looking at VectorVest, as a replacement for (or in addition to) Incredible Charts, as it has very powerful backtesting routines and the ability to run test portfolios with specific buy/sell criteria that can simulate and backtest a number of trading scenarios at the same time. The advantage of all of these is they are not tied to a particular broker. Any time you are optimizing a portfolio, the right horizon to use for computing the statistics you will use for optimization (expected return, covariance, etc.) will be the same as your rebalance/trading frequency.  If you expect your trading strategy to trade once a day, you should use daily data for optimization.  Ditto for monthly or quarterly.  If at all possible you should use statistics across the board that are computed at the same frequency as your trading. Regarding currency pricing, I see no reason you can't take the reported prices and convert them to whatever currency you want using that day's foriegn exchange rate.  Foreign exchange rates are available for free at the Fed and elsewhere.  Converting prices from one currency to another is not rocket science. Since you are contemplating putting actual money behind this, note that using data to compute statistics is less reliable for lower statistical moments.  The mean (expected return) is the first moment, so using historical returns is extremely unreliable at predicting future returns.  The variances and covariances are second moments, they are better.  Skewness and kurtosis, yet better.  The fact that the expected return can't reliably be estimated from past returns is the major downfall of the Markowitz method (resulting portfolios are often very crazy and will depend critically on the data period you use to set them up).   There are approaches to fixing this, such as Black-Litterman's (1992) method, but they get complicated fast. I use and recommend barchart.com. Again you have to register but it's free. Although it's a US system it has a full listing of UK stocks and ETFs under International > London. The big advantage of barchart.com is that you can do advanced technical screening with Stochastics and RS, new highs and lows, moving averages etc. You're not stuck with just fundamentals, which in my opinion belong to a previous era. Even if you don't share that opinion you'd still find barchart.com useful for UK stocks. you need to use easy programming language to imply onto a scan  where you enter Scan all stocks  display if volume < (less than) 100