You go to your doctor to get a check-up.
A nurse wraps your arm with a pressure cuff and pumps it up to measure your
blood pressure which is recorded as two numbers maybe 114/74. So far, so good.
But what if you want to use that blood pressure reading in a statistical
analysis? You can’t enter 114/74 because it
will be treated like a ratio of 150% We need a
single number that is usable for statistics. For this example, we’re going
to use both Excel and SPSS as we dive deeper into how to solve the tricky
problem of isolating the data that we need computing a new variable with a
formula and then creating a single number called the mean arterial pressure. For this example we’re going to start
with some data in Excel. Now these data come from patient charts and they
represent blood pressure readings for a collection of patients. We’re going to
isolate the parts of the data that we need, and then use a formula to assemble
those parts into a meaningful variable that we can use to measure blood
pressure. We are going to accomplish this in three steps. Step 1: separate the
values in Excel using the LEFT and RIGHT commands.
Step 2: bring that data into SPSS. Step 3: calculate a new variable for MAP
using the formula. Here is why we are starting this process in Excel: I don’t
know of any way to do this in SPSS. In Excel, however, we can use the LEFT and
RIGHT functions to isolate each part of the blood pressure reading. You, of course,
recognize these as blood pressure readings: systolic over diastolic. But
Excel sees them as ratios, like 1.52% We want the systolic
numbers in one column and the diastolic numbers in a second column, so that we
can then use them in the mean arterial pressure formula.=((2*DIA)+SYS)/3 And where did I get this formula? The formula for mean
arterial pressure is in most nursing textbooks or you could find it with a
quick internet search. We will begin by creating some new
columns labeled SYS for systolic and DIA for diastolic.
For the systolic numbers, we will use the LEFT function. The syntax for the left
function is this you first reference the cell that contains the text, followed by
the number of characters. The systolic numbers are the first three numbers on
the left so we would type=LEFT (A2, 3). This creates a cell with the first three numbers on the left end.
Highlight the cell and double-click on the handle to copy it to the remaining
cells. If all of the diastolic numbers are two digits, then the next part is
easy. We simply use the RIGHT function to copy only the rightmost two digits into
the DIA column. This creates a cell with the two numbers on the right end.
Highlight the cell and double click on the handle to copy it to the remaining
cells. In our case however, we have several blood pressure readings with
three diastolic digits. Some have two digits, some have three digits. What can
we do? The solution is to improve our diastolic formula. For the diastolic
numbers we will still use the RIGHT formula, referencing cell A2, but for the
number of values we have Excel return the entire length of A – – everything
before the slash using the SEARCH function. Double click to copy down. LEFT and RIGHT
functions belong in the category of TEXT functions in Excel, meaning that the
result of a LEFT or RIGHT formula is always a text string, even if the
original values were numbers; therefore, these numbers are being read as strings.
So if you try to do the mean arterial pressure calculations in Excel, you will
get an error. That is why at this point I import these values into SPSS. After
import, SPSS will read the numbers as numeric values and import them that way.
But for those of you who are wanting to do this entirely in Excel, you should
wrap the LEFT or RIGHT functions in the VALUE function, which is designed to
convert a string into a number. I am going to save this Excel file to
the desktop, so that I can import it into SPSS. In SPSS I go to File -> Import Data ->
Excel. I go to the Desktop, select the blood pressure file and click Open. I click OK in the import dialog box and
the file opens. Notice that both systolic and diastolic numbers have imported as
scale data. To create the mean arterial pressure, I will use the Compute Variable
function with a formula. Go to Transform ->->Compute Variable. I will create a
variable called MAP, and for the variable label, I will type “Mean Arterial
Pressure”. The formula for MAP is ((2*DIA)+SYS)/3. Click OK. We now have a single number calculated from blood
pressure readings that we can use for analyzing blood pressure among our
respondents.